Risk factors for male breast cancer.

To investigate risk factors in male breast cancer, a case-control study of 52 histologically diagnosed cases and 52 controls--matched for age, race, marital status, and hospital--was conducted in 5 U.S. metropolitan areas. Cases were significantly more likely to be Jewish than were the controls, supporting earlier suggestions of an increased risk in Jewish males. A significant association of male breast cancer with mumps infections at age 20 years or older, along with the possible association with antecedent testicular injury and the excess frequency of mumps orchitis among cases, suggests that testicular factors may be important in the development of breast cancer among males. An increased frequency of breast cancer among persons who have worked in blast furnaces, steel works, and rolling mills is of interest because of the possible testicular effect of high environmental temperatures. The observed association between breast cancer and a prior history of swollen breast is difficult to interpret because of potential recall bias, and a possible relationship with military service needs further confirmation.