A prospective cohort study of oral contraceptives and cancer of the endometrium.

A prospective cohort study of approximately 97 300 women was conducted in eastern Massachusetts. Information on use of oral contraceptives, and on gynaecological and reproductive histories was sought through mailed questionnaires, while newly diagnosed cases of cancer of the endometrium were identified from records of 34 area hospitals. Oral contraceptive (OC) use was associated with some increase in endometrial cancer rates (standardized rate ratio = 1.4; 95% confidence interval = 0.9-2.4), although the risk did not vary consistently by length of use, years since first use, or age at initial use. Although some subgroups of users had rates of disease higher than among nonusers, there was little to suggest a consistent association between OC use and incidence of endometrial cancer.