We report a population-based case-control study on risk factors for male breast cancer. Data on a broad range of previously suggested risk factors were collected in a set of Scandinavian breast cancer cases and matched controls. Incident cases (n = 282) with histologically verified carcinomas of the breast were identified from notification to the cancer registries of Denmark, Norway and Sweden over a 4-year period 1987-1991 and of these cases, 156 men could be approached and responded. Controls were identified through national central population registers and were matched individually for country, sex and year of birth. Controls with a diagnosis of breast cancer were excluded; 468 of 780 controls responded. Data on risk factors were collected by self-administered questionnaires mailed to the cases between land 2 years after diagnosis and to controls during the same period. The findings were compatible with an increased risk associated with family history of breast cancer (odds ratio (OR) = 3.3, 95% confidence interval (CI) 2.0-5.6), obesity 10 years before diagnosis (OR = 2.1, 95% CI 1.0-4.5) for BMI > 30, diabetes (OR = 2.6, 95% CI 1.3-5.3) and the use of digoxin and methyldopa (OR = 2.0 and 2.1, respectively). The association with family history of breast cancer has been repeated in several studies, while the relation to anthropometric measures has been equivocal. We could not substantiate some associations seen in other studies; namely those with high education, fertility, marital status, testicular injury, liver disease and religion. The detailed questions about gynaecomastia indicated that many cases reported signs of breast cancer as a gynaecomastia. This type of misunderstanding may explain the strong association with gynaecomastia seen in other studies. Several patients died before contact. Thus, risk factors related to a more aggressive male breast cancer or related to high risk of dying (e.g. liver cirrhosis, heavy smoking) may have been missed.