In a hospital-based case-control study of the epidemiology of breast cancer undertaken in Connecticut from 1977 to 1979, there was no evidence of an increase in risk for breast cancer among women who had used oral contraceptives or estrogen-replacement therapy. In fact, there was some suggestion of a decrease in risk for breast cancer with increasing length of use of oral contraceptives. Higher than average risks were found among women who had never give birth to a child, women with a late age at menopause, women, with an early age at menarche, women who had given birth to their first child at a relatively late age, women with previous benign breast disease, and women with a history of breast cancer in a sister or mother. Heavy women were at high risk for premenopausal breast cancer. The association between heaviness and postmenopausal breast cancer was strongest among women who had had their last menstrual period more than 5 years before the diagnosis of breast cancer.