Breast cancer is an important cause of morbidity and mortality in Brazil. Some studies have analyzed the potential role of organochlorine compounds in breast cancer etiology. These chemical compounds have been widely used in agriculture and in vector-control programs in Brazil. A case-control study was carried out in the main cancer hospital of the Instituto Nacional de Câncer in Rio de Janeiro to investigate exposure to organochlorinated pesticides as a risk factor for breast cancer. We investigated 177 cases of invasive breast cancer at the hospital, between May 1995 and July 1996, and 350 controls selected among female visitors at the same hospital. In addition to information obtained from interviews, blood samples were taken, to analyze residual amounts of organochlorine by gas chromatography using an electron capture detector. [1,1-Dichloro-2,2-bis(p-chlorophenyl)ethylene] (DDE) was determined in sera of 457 women from a total of 493 participants who had serum samples available. Residues of hexachlorobenzene were found in 11 women only. No statistically significant association was found between breast cancer risk and serum level of DDE or history of exposure to pesticides. Serum levels of DDE (ng/ml) were similar in patients (median = 3.1, mean = 5.1) and controls (median = 3.1, mean = 4.8) (p = 0.93). The age-adjusted odds ratio of breast cancer for women in the upper quintile compared with those in the lowest quintile was 0.90 (95% confidence interval 0.47–1.73). When adjusted for other variables, the risk remained not statistically significant (upper quintile odds ratio = 0.79, 95% confidence interval 0.39–1.60). In our hands, exposure to organochlorinated pesticides measured by history or serum analysis was thus not a risk factor for breast cancer. Int. J. Cancer 83:596–600, 1999. © 1999 Wiley-Liss, Inc.