The incidence of breast cancer among users and non-users of oral contraceptives (OCs) was determined at Group Health Cooperative of Puget Sound, Seattle, Washington, a health care organization which maintains computer files of diagnoses and outpatient drug use. In women 45 years of age or younger, the incidence was nearly identical in users and non-users. In premenopausal women over 45 years of age, there was a positive association between current OC use and breast cancer, the risk ratio estimates and 90% confidence intervals being 4.0 (1.8–9.0) in women 46–50 years of age and 15.5 (5.2–46) in women 51–55 years of age.
