PIP: An attempt is made to summarize the results of recently conducted epidemiological studies relating to the possible adverse effects of oral contraceptives (OCs) on breast cancer. Evaluation of the safety of OCs is difficult. An important reason is that since preparations were introduced to the American market in 1959 the formulation of pills and the dosage of constituents have both changed markedly. A number of other considerations to be born in mind when evaluating the relevant literature were identified by a recent World Health Organization (WHO) Scientific Group. These include the following: there is usually a considerable latent period between 1st exposure to a carcinogen and the development of overt malignancy; and it is possible that exposure to contraceptive steroids may be particularly important at certain critical periods during reproductive life. After age, the most important risk factors during early life are a young age at menarche and a late age at 1st full term birth. There factors appear to operate independently and so the longer the period between menarche and 1st full term birth, the higher is the risk of breast cancer. A girl whose menarche occurs before age 12 is approximately twice as likely to develop breast cancer later in life as a girl whose menarche occurs after age 14. Similarly, a woman whose first full term birth occurs after age 35 is about 3 times more likely to develop breast cancer than a woman who gives birth before age 20. Nulliparous women have an intermediate risk. Other known risk factors include a family history of breast cancer, a past history of benign breast disease, and increased body weight. Possibly the most serious problem from an epidemiological perspective is the interrelationship between calendar time, age, and OC exposure. OC seems to be clearly associated with a decreased risk of benign breast disease of sufficient severity to require biopsy and the evidence is that the reduction in risk increases with duration of use. The epidemiological evidence available regarding OCs and breast cancer is reassuring. Data have been obtained from case control studies and cohort studies. A table summarizes the main features of 12 case control studies. Results of both groups of studies are reassuring in relation to breast cancer, yet there is usually a long period between exposure to a carcinogen and the development of overt malignancy and OCs have been in widespread use for only a relatively short time. Women who have never used OCs seem to present with more clinically advanced tumors than women who have used them. The differences in staging are reflected in patterns of survival. The difference may be due to a greater awareness of breast disease in OC users, but it could represent a beneficial biological effect of OCs.