An analysis of 489 breast cancer pedigrees was conducted to determine the age at diagnosis and bilaterality rate in 887 patients with different familial patterns of the disease compared with 5,100 unselected patients, and to estimate the lifetime probabilities of breast cancer development in 983 daughters of familial patients. The familial patients had higher rates of bilaterality and were younger at diagnosis than patients in an unselected series. Bilaterality rates were highest in young patients. Familial bilateral patients developed their first primaries about 5 years earlier than unilateral patients. Daughters of patients with any type of family history of breast cancer exhibited a 23% lifetime probability for the disease. The highest probability (27%) pertained to daughters of patients whose mothers were also affected, and it was concluded that this group referred to a hereditary type of breast cancer distinct from types involving associated neoplasms. The occurrence of multiple neoplasms characterizes the hereditary types of breast cancer.