The possibility of a genetic relationship between ovarian, breast, and endometrial cancer was investigated in data from a large multicenter, population-based, case-control study, the Cancer and Steroid Hormone Study conducted by the Centers for Disease Control (CDC). Age-adjusted relative risks (RRs) for mothers and sisters of 493 ovarian cancer cases, 895 breast cancer cases, and 143 endometrial cancer cases versus 4,754 controls were calculated. Significantly elevated age-adjusted RRs were found for ovarian cancer (RR = 2.8; 95% confidence interval [CI] = 1.6-4.9) and breast cancer (RR = 1.6; 95% CI = 1.1-2.1) among relatives of ovarian cancer probands and for breast cancer (RR = 2.1; 95% CI = 1.7-2.5) and ovarian cancer (RR = 1.7; 95% CI = 1.0-2.0) among relatives of breast cancer probands. Relatives of endometrial cancer probands had an elevated RR for endometrial cancer only (RR = 2.7; 95% CI = 1.6-4.8). The genetic relationship between ovarian, breast, and endometrial cancer was tested using a multivariate polygenic threshold model developed by Smith (1976), which was modified to accommodate three classes of probands. Estimates of heritability for ovarian, breast, and endometrial cancer were 40%, 56%, and 52%, respectively. There was a significant genetic correlation between ovarian and breast cancer (R12 = .484). Evidence for significant genetic overlap between endometrial cancer and either ovarian or breast cancer was not found. These results suggest the existence of a familial breast/ovarian cancer syndrome. Endometrial cancer, while heritable, appears to be genetically unrelated.