Risk factors for male breast cancer were investigated in a case-control study of 21 cases and 82 controls admitted to hospital for acute, non-neoplastic, non-hormone-related diseases in the Greater Milan area between 1988 and 1994. More educated men tended to be at higher risk of breast cancer, with a multivariate odds ratio (OR) of 2.6 [95% confidence interval (CI) 0.7-9.4]. The OR was 3.2 (95% CI 1.1-9.6) for those in the higher social class. Men with no offspring were at higher risk than fathers, with an OR of 5.5 (95% CI 1.8-16.7). A history of breast cancer in female relatives was reported by two cases and one control, giving an OR of 8.5 (95% CI 1.1-69.0). Cases were somewhat heavier than controls, and significantly taller, with an OR of 5.7 (95% CI 1.6-19.9) for subjects taller than 170 cm vs shorter ones. The association with weight, however, decreased after allowance for height, and no difference was observed for body mass index. Socioeconomic correlates and family history are similar to well-assessed risk factors for female breast cancer. The associations with anthropometric measures and childlessness may find an explanation in chromosomal abnormalities, such as Klinefelter's syndrome, or other hormone-related disorders.