Previous breast carcinoma in at least one female relative was reported by 31% of 1024 women treated consecutively for breast carcinoma at Memorial Hospital. Eighty (7.9%) of their mothers had had breast cancer. Maternal breast cancer was significantly (P less than 0.006) more frequent among women with medullary carcinoma than those with other tumor types and among those who were pre- or perimenopausal at diagnosis (P less than 0.001). Among the 727 patients who had one or more sisters, 12% had a sister who had been treated for breast cancer. The highest frequency of carcinoma in at least one sister occurred in patients with lobular carcinoma while the medullary carcinoma group had the least number of patients with an affected sister (P less than 0.03). Occurrence of breast cancer in a sister was almost twice as common in patients who were postmenopausal at diagnosis (P less than 0.005) than in premenopausal patients. When stratified by histologic type, the mean age at diagnosis of the patients did not differ appreciably from the age at diagnosis of their sisters. Detailed analyses of histologic type and other more distant familial relationships were also obtained but were considered to be less reliable because of problems in ascertainment and there were fewer affected relatives. No single histologic type of carcinoma was consistently linked to a disproportionately high or low frequency of carcinoma in all classes of relatives. It is possible that studies of family history limited to information available when the patient is first treated present an incomplete picture of familial aggregation. Further follow-up after diagnosis is needed to obtain a more reliable measure of the extent to which relatives are affected by breast cancer and patterns of family distribution associated with specific tumor types.