A case-control study of the relationship between use of oral contraceptives and cancer of the breast, cancer of the corpus uteri, cancer of the ovary, and benign breast diseases was undertaken among women in the age group 20--44 years who had been admitted to several large hospitals in Connecticut. No significant association was seen between use of oral contraceptives and breast cancer. It was found that the longer a woman had used sequential oral contraceptives, the lower her relative risk of benign breast disease. Use of combination oral contraceptives, however, did not appear to influence the risk of benign breast diseases. The numbers of cases with cancer of the ovary and of the corpus uteri in this age group were too small for any conclusions to be reached.