Twelve pedigrees which show clustering of breast/ovarian cancer among female relatives are analyzed from a medical-genetic standpoint. The significant linear decline in estimates of cumulative breast/ovarian cancer risk to females with diminishing genetic relationship to probands and index cases, plus the lack of excessive site-specific cancer risk to male relatives, supports a sexlimited genetic etiology. Breast/ovarian cancer typically occurred at an early age (2 = 50.6 years) in these kindreds compared to the general population, which is another characteristic of hereditary cancer. Examples of apparent mother to daughter and father to daughter genetic transmission of proclivity for breast/ovarian carcinoma were prevalent in the 12 pedigrees. Excluding probands and index cases, the estimated cumulative risk of breast/ovarian cancer to female progeny of affected mothers was 46% for the age interval 20-80 years, suggesting that affected mothers in the pedigrees are transmitting a deleterious cancer-predisposing gene to one-half of their daughters. Eight sibships involving putative carrier males contain a total of 15 female progeny over age 20. Of these, six have manifested breast cancer and five have had ovarian cancer. These results underscore the need for physicians to be cognizant of both males and females as potential transmitters of cancer in this familial tumor association.