The prevalence of a family history of breast cancer was established in 54 women with bilateral primary breast cancer and 208 women with unilateral disease. Women with bilateral disease had significantly greater prevalence of family history than women with unilateral breast cancer (P less than 0.01). Compared with the unilateral cancers, a significantly greater proportion of bilateral cancers had first degree affected relatives (P less than 0.05). Moreover the affected relatives of probands with bilateral disease showed a significantly higher prevalence of bilateral breast cancer compared with the relatives of women with unilateral disease (P = 0.04). The findings suggested that bilateral disease was a characteristic of familial breast cancer.