The relationship between spontaneous or induced abortion and the risk of breast cancer was analyzed in a case-control study conducted in the greater Milan area on 2,394 cases of breast cancer and 2,218 controls in hospital for a spectrum of acute conditions, not gynecological, hormonal or neoplastic. No consistent relationship emerged between spontaneous or induced abortion and breast cancer: compared with women reporting no abortions (spontaneous or induced), the multivariate relative risk (RR) was 1.0 (95% confidence interval, CI, 0.9 to 1.2) in those reporting one abortion and 0.9 (95% CI 0.7 to 1.0) in those reporting two or more. This lack of association was consistent in strata of age and parity, including younger women. We further analyzed the risk of breast cancer associated with an abortion before and after full-term pregnancy. Compared with parous women reporting no induced or spontaneous abortions, those who had an abortion before their first full-term pregnancy had about a 20% higher risk of breast cancer. This finding, however, was not statistically significant (RR 1.2, 95% CI 0.9 to 1.7). No increased risk was observed in women who had had a first abortion after a full-term pregnancy (RR 0.9, 95% CI 0.8 to 1.0). This study does not support the hypothesis that spontaneous or induced abortion appreciably influences subsequent breast-cancer risk.