Weuve et al. Wilcox et al. 1994) and are associated with significant costs for lost productivity and hos- pitalization, accounting for several billion dol- lars annually in the United States for health care alone (Flynn et al. 2006; Simoens et al. 2007). Although the etiologies of both condi- tions remain poorly understood, there is evi- dence that they are influenced by endogenous sex steroid hormones (Eskenazi and Warner 1997; Schwartz et al. 2000). Four epidemiologic studies have assessed phthalate exposures in relation to these condi- tions, and their findings suggest increased risk with higher levels of exposure to some phtha- lates (Cobellis et al. 2003; Itoh et al. 2009; Luisi et al. 2006; Reddy et al. 2006). However, these studies were small (with 15–57 cases), adjusted for few if any potential confounders, and recruited controls from clinical settings, a potential source of bias. Moreover, most of these studies relied on plasma or serum levels of phthalate diesters and monoesters as bio- markers of exposure. These exposure measures have two critical limitations. First, measure- ments of diesters in any biologic medium are prone to contamination from phthalate- containing laboratory supplies and other sources. Second, monoester measurements in blood are also susceptible to contamination, because serum enzymes can hydrolyze diesters in the blood to their respective monoesters after the specimen has been collected (Kato et al. 2003). The potential for contamination generally decreases when measuring phthalate monoester metabolites in urine. In assessing exposure to DEHP, the potential for contami- nation is virtually eliminated when measuring the metabolites produced via enzymatic oxida- tion (Kato et al. 2004). Therefore, we examined the association of phthalate exposures with endometriosis and uterine leiomyomata, using cross-sectional data on urinary phthalate metabolite concentrations and self-reported reproductive history from 1,227 women participating in the National Health and Nutrition Examination Survey (NHANES), 1999–2004. In addition, for 838 women, we examined urinary concentrations of two oxidative metabolites of DEHP, which were measured in NHANES 2001–2004. Materials and Methods We used data from three cycles (1999–2000, 2001–2002, 2003–2004) of NHANES [Centers for Disease Control and Prevention (CDC) 2009] to examine the association between urinary phthalate metabolites and previously diagnosed endometriosis or uterine leiomyomata. Briefly, NHANES is an ongo- ing cross-sectional national survey conducted by the National Center for Health Statistics at the CDC, designed to collect data on the health and nutritional status of the civilian, noninstitutionalized U.S. population. Data 826 estimates are sampling-probability based and are representative of the U.S. population. NHANES collects data through question- naires, physical examinations, and laboratory tests. The assessment includes both a house- hold interview and mobile examination com- ponent (MEC), during which blood and urine are collected. Sociodemographic information and medical histories of the survey participants and their families were collected during the household interviews (CDC 2009). All par- ticipants provided written informed consent, and the NCHS obtained institutional review board approval to conduct the survey. Study population. NHANES 1999–2004 was conducted in 87 locations throughout the United States and included examinations of 31,126 people of all ages (15,942 females). A random one-third subset of participants was selected by NHANES for phthalate metabolite measurements, thereby retaining the represen- tational aspect of these data (CDC 2009). Women participating in the MEC assess- ment and who completed the reproductive health questionnaire (given to all female par- ticipants ≥ 12 years of age) were eligible for our analyses (n = 10,946). Questions pertaining to endometriosis and leiomyomata were restricted to women 20–54 years of age, further restrict- ing the eligible population to 4,094 women. Of these women, 1,379 were included in the one-third subset of participants whose urinary specimens were analyzed for phthalates. We further excluded the 127 women whose uri- nary creatinine concentrations were < 30 or > 300 mg/dL (Silva et al. 2004). An additional 25 women were missing data on key covariates, leaving 1,227 women for our analyses of endo- metriosis. Of these women, six were missing responses pertaining to uterine leiomyomata, and thus our analyses of this outcome included 1,221 women. Because menopausal status is unlikely to influence the measurement of uri- nary phthalate metabolites, we retained women who were postmenopausal at the time of the MEC interview. Our analyses of two oxidative metabolites of DEHP (assessed in NHANES 2001–2004) in association with endometriosis and uterine leiomyomata, included 838 and 835 women, respectively. Women who were excluded from our main analyses did not differ substantially from included women with respect to a range of demographic, lifestyle, and reproductive health factors. For example, these groups were similar in age (35.6 vs. 35.7 years), age at menarche (12.6 vs. 12.6 years), parity status (25% vs. 27% nulliparous), and race/ethnicity (19% vs. 21% African American). Assessment of exposure to phthalates. We examined the metabolites of four prevalent phthalates measured in the three consecu- tive NHANES cycles: MEHP, a monoester metabolite of DEHP; MBP; MEP, metabolite of diethyl phthalate (DEP); and monobenzyl phthalate (MBzP), the main metabolite of BzBP. In the 2001–2002 and 2003–2004 cycles, concentrations of mono-n-butyl phtha- late (MnBP) and mono-isobutyl phthalate were measured separately, and we summed these concentrations to obtain total MBP concen- tration for these cycles. Data for other phtha- late metabolites—monocyclohexyl phthalate, mono-n-octyl phthalate, and monoisononyl phthalate—were also available for the three NHANES cycles. However, these metabolites were detected in < 10% of the samples exam- ined and were not included in our analyses. Urinary concentrations of additional phtha- late metabolites were introduced in the 2001– 2002 cycle. Of particular interest to our study were concentrations of two oxidative metabo- lites of DEHP: mono(2-ethyl-5-hydroxyhexyl) phthalate (MEHHP) and mono(2-ethyl-5- oxohexyl) phthalate (MEOHP). These metabo- lites are produced only via enzymatic oxidation and are not produced in the environment. Moreover, their concentrations in urine are often at least an order of magnitude greater than concentrations of MEHP, making them potentially more sensitive measures of expo- sure to DEHP. Therefore, we also examined these two oxidative metabolites for data in the 2001–2002 and 2003–2004 cycles to evaluate the consistency of the findings for MEHP. As a summary measure of DEHP exposure, we com- puted the molar sum concentration of MEHP, MEHHP, and MEOHP. In exploratory analy- ses, we evaluated a fourth metabolite of DEHP, the oxidative metabolite, mono(2-ethyl-5- carboxypentyl) phthalate (MECPP), which was measured only in the 2003–2004 cycle. Urine specimens for analyses, including phthalate metabolites and creatinine concen- trations, were collected from each participant during one of three daily examination peri- ods (0830–1200, 1230–1600, or 1630–2000 hours) (Silva et al. 2004). After collection, urine specimens were aliquoted and stored cold (2–4°C) or frozen until they were shipped to the various laboratories for analysis (Silva et al. 2004). Urine specimens were analyzed for creatinine using a Beckman Synchron AS/ASTRA clinical analyzer (Beckman Instruments, Inc., Brea, CA) at the University of Minnesota Medical Center (Silva et al. 2004). Samples collected for phthalate metab- olite measurements were shipped on dry ice to the CDC’s National Center for Environmental Health. Urine samples were stored frozen at or below –20°C until analyzed. The samples were analyzed by solid-phase extraction followed by isotope-dilution high-performance liquid chro- matography and tandem mass spectrometry as previously described (Blount et al. 2000a; Silva et al. 2003). To correct for urine dilution, we divided all phthalate metabolite concentrations by urinary creatinine concentration (Jackson ovlume 118 | number 6 | June 2010 • Environmental Health Perspectives