There is concern that persistent environmental pollutants such as dichlorodiphenyltrichloroethane (DDT) and polychlorinated biphenyls (PCBs) increase breast cancer risk, at least partially through estrogenic effects. Because the endometrium is more sensitive to estrogenic stimulation than the breast, such a carcinogenic effect should be more pronounced in the endometrium than the breast. In a population-based case-control study in Sweden, we measured serum concentrations of 10 chlorinated pesticides and 10 PCB congeners in 154 endometrial cancer cases and 205 population controls. Information on potential confounders was obtained by mailed questionnaires. We used logistic regression to calculate odds ratios (ORs) as measures of relative risk. We performed analyses for lipid-adjusted concentrations of each individual substance and after grouping substances according to putative hormonal effects. We found no significant associations of increasing levels of pesticide or PCB exposure with endometrial cancer risk. The multivariate OR was 1.0 (95% confidence interval, 0.6-2.0; P for trend, 0.78) for the highest compared with the lowest quartile of dichlorodiphenyldichloroethylene (DDE), the predominant dichlorodiphenyltrichloroethane metabolite. Corresponding ORs were 1.0 for hexachlorobenzene, 0.9 for beta-hexachlorocyclohexane, 1.4 for oxychlordane, and 1.2 for trans-nonachlor. Analyses of substances grouped by putative hormonal effect also showed no associations with endometrial cancer risk. For all estrogenic compounds, the OR for the highest compared with the lowest quartile was 1.1 (95% confidence interval, 0.6-2.2; P for trend, 0.90). Our data do not support the hypothesis that the organochlorine exposure studied increases the risk for endometrial cancer.