['The possible role of heme oxygenase and its byproduct carbon monoxide (CO) in the regulation of blood pressure is under investigation. The aim of this study was to compare end tidal breath CO (ETCO) levels in women with gestational hypertension (GH) or pre-eclampsia to the levels in healthy pregnant and nonpregnant women.', 'We prospectively performed ETCO measurements corrected for ambient CO (ETCOc) in two medical centers (Stanford, CA and Cleveland, OH). A Natus CO-Stat End Tidal Breath Analyzer (Natus Medical Inc., San Carlos, CA) was used. The study group included a convenience sample of 31 women with GH/pre-eclampsia (PE). Control groups included 46 nonpregnant healthy women, 44 first-trimester and 48 third-trimester pregnant healthy women.', 'Mean+/-SD ETCOc measurements were significantly lower in the GH/PE group compared to first-trimester (p=0.004) and third-trimester (p=0.001) normotensive pregnant and nonpregnant women (p=0.002) (1.36+/-0.30 vs 1.76+/-0.47, 1.72+/-0.42 and 1.78+/-0.54 ppm, respectively). The ETCOc values were < or =1.6 ppm in 89% of GH/PE women compared with, respectively, only 45, 54, and 46% of nonpregnant, first- and third-trimester normotensive pregnant women (p<0.05). ETCO measurements were not influenced by maternal age, parity, ethnicity, body mass index, gestational age or presence of household smokers. In the two centers, the controls had a similar mean ETCOc and the differences found remained significant when results for each center were analyzed separately.'] Are end tidal carbon monoxide levels lower in women with gestational hypertension and pre-eclampsia?