exposure to DDT. However, it has been proven that DDT causes hepatic alterations in various rodent species and that these changes can progress to tumor formation in some species (WHO, 1979). Although several studies have attempted to correlate DDT exposure and cancer in humans, none has obtained unequivocal confirmation of this correlation (Albert, 1981; Cantor & Booze, 1991; Falck et al., 1992, Olaya-Contreras et al., 1998, Romieu et al., 2000; Wassermann et al., 1979). Another more recent concern is the potential of DDT and its metabolites to interact with the endocrine system (McLachlan & Arnold, 1996). The effects of these substances on reproduction in birds and alligators have already been shown. Snedeker (2001) reviewed studies which attempted to correlate pesticides (DDT, DDE, and dieldrin) with breast cancer risk. According to the author, several early descriptive studies and a cohort study identified a positive association between breast cancer risk and adipose or blood levels of these pesticides. Most of the nested case-control and case-control studies conducted since 1996 failed to confirm these early observations, while two studies conducted in Colombia and Mexico found an increased risk of breast cancer in women with high DDE levels. Malaria incidence was high in the northern region of Mato Grosso State, along the southern border of the Amazon Forest, where high amounts of DDT may have been used. The Brazilian National Health Foundation divides the State into four districts for administrative purposes (Figure 1). The District of Sinop, located in the North of Mato Grosso, and that of Caceres, in the Southwest of the State, were selected for this study, aimed at obtaining data on contamination levels in National Health Foundation (FUNASA) personnel involved in household DDT spraying. Methods The District of Sinop, in the North of Mato Grosso, includes 30 municipalities (or counties). The main economic activities are lumbering, cattle-raising, and gold-mining. The FUNASA staff in Sinop consisted of 144 sprayers (also referred to as health agents), 15 drivers, and 15 office workers. In the District of Caceres, in the Southwest of Mato Grosso, with 29 municipalities, there were 74 sprayers, 13 drivers, and 8 office workers at the time of the study. Since there were no previous data on serum DDT levels in Mato Grosso, the sample size was calculated based on data from the study carried out by Minelli & Ribeiro (1996a) with sprayers in Sao Jose do Rio Preto, Sao Paulo. In addition to the sprayers, drivers were also considered to be exposed to DDT, since they were responsible for loading and unloading the insecticide in the vehicles, besides accompanying the house spraying. Office workers were classified as unexposed. The samples collected and analyzed in June 1999 in Sinop consisted of 23 sprayers, 12 drivers, and 7 office workers, and those collected in October 2000 in Caceres included 18 sprayers, 8 drivers, and 7 office workers. Blood sample donors were randomly chosen from the staff list furnished by FUNASA/MT, and participation in the study was voluntary. After explaining the study's objectives and procedures, the randomly chosen volunteers were asked to sign an informed