In order to evaluate risk factors in male breast cancer, a case-control study of 91 histologically diagnosed cases and 255 cancer controls, matched for age and year of diagnosis, was conducted in Villejuif (France) and Geneva (Switzerland). Factors studied included marital history, occupational exposure, associated diseases, family history of breast cancer, history of radiation exposure and drugs which increase or are suspected of increasing prolactin or estrogen levels. Cases were significantly more likely to be bachelors or to work as butchers than were the controls. Other statistically significant risk factors identified were family history of breast cancer and therapeutic use of digitalis. Elevated risks, although not significant, were observed for individuals with a past history of tuberculosis, hyperthyroidism, liver cirrhosis or gallstone, who had been treated with isoniazid, or who had worked in high environmental temperatures. Further studies are needed to confirm our results particularly as regards hormonal imbalance and genetic factors.