Two groups of pedigrees were compared, one pertaining to cancer patients with family histories of neoplasms other than those of the breast (controls), and the other to breast cancer patients with family histories of breast cancer. First-degree relatives of breast cancer patients, excluding the affected relatives who led to ascertainment of these patients, manifested a twofold to threefold higher breast cancer frequency than control relatives, a type of finding observed in past studies. The risks increased from 1.8–3.1 for relatives of patients with premenopausal diagnoses compared with control relatives, but no increase in risk was observed for relatives of patients with postmenopausal diagnosis. When the disease in the patient was bilateral, the risk for their relatives was fivefold in excess of controls, and if both premenopausal and bilateral, the risk was ninefold. The risks for relatives of patients with unilateral disease, whether premenopausal or postmenopausal, ranged from 1.2–1.8, only slightly higher than control values. Relatives of bilateral patients manifested a ninefold higher risk of bilateral cancer itself than did relatives of unilateral patients. These results suggest that genetic factors are more important in patients with early onset of multiple disease than in patients with late onset of a single tumor. Moreover, the well-known twofold to threefold risks generally cited for relatives of breast cancer patients may not always apply, and may well be much too low, particularly if the disease is familial, premenopausal, and involves multiple tumor foci.