Several studies have investigated the possible relationship between fertility drugs and the risk of breast cancer. To provide further information on this issue, we analysed data from a case control study, conducted in Northern Italy between 1983 and 1991. Trained interviewers identified and questioned 3415 cases (women aged 23-74 years with histologically confirmed breast cancer) and 2916 controls (women aged 21-74 years admitted to the same hospitals for diseases other than malignant, hormonal or gynaecological conditions). Fifty (1.5%) cases and 53 (1.8%) controls reported any history of infertility; the corresponding multivariate odds ratios (OR) of breast cancer was 0.8 [95% confidence interval (CI) 0.5-1.1]. Sixteen (0.5%) cases and 11 (0.4%) controls reported ever using fertility drugs; the corresponding OR was 1.2 (95% CI 0.5-2.6). Allowance for potential confounding factors did not markedly modify these estimates. In conclusion, this study provides reassuring evidence on the absence of an association between fertility drug treatment and breast cancer risk.