The family history of breast cancer was studied in 1975 breast cancer patients of all ages. All the patients were found in an ongoing care program for breast cancer patients in the Stockholm area. One hundred and thirty-three families (6.7%) with hereditary breast cancer, 87 (4.4%) families with hereditary breast- and other cancers, and 14 (0.7%) families with the SBLA syndrome were found. There was no association between familiarity and bilateral disease, but familial cases tended to be younger at the time of diagnosis. Determination of the parental origin of the disease gene among the index cases revealed twice as many cases where the disease was maternally transmitted. This might be due to a better prognosis when the predisposing gene is maternally transmitted, and could be explained by parental imprinting or environmental factors influencing the expression of the gene.