The relation of oral contraceptive use to the risk of breast cancer was evaluated in a case-control study of women under the age of 70 years, conducted in Toronto, Canada, from 1982 through 1986. A total of 607 breast cancer cases identified in a cancer hospital were compared with 1,214 controls matched to the cases on neighborhood and decade of age. Information on oral contraceptive use and risk factors for breast cancer was collected in home interviews. Conditional logistic regression was used to control multiple confounding factors. For women aged 40-69 years (527 cases, 1,054 controls), the results suggest that oral contraceptive use does not increase the risk of breast cancer. Multivariate relative risk estimates were close to or below 1.0 for long durations of use overall and in various categories of parity status and other factors. For women under the age of 40 years, the data were sparse (80 cases, 160 controls). Although there were some elevated relative risk estimates, most were not statistically significant, and there were no consistent patterns across duration of use. The present data add to the body of evidence that indicates that oral contraceptive use does not adversely affect the risk of breast cancer in older women; the data are inadequate to clarify the effect in younger women.