Hereditary ovarian cancer (HOC) is rare and little recognized. Over the years, we have identified 37 HOC patients from HOC syndrome kindreds with documented cancers of ovary, breast, colon, or endometrium in two or more first-degree relatives. The age and clinical stage at diagnosis and overall 5-year survival of HOC patients were compared with those of ovarian cancers in the unselected patients. The gross and microscopic features of the tumors are compared with a set of 34 consecutively chosen ovarian cancer cases with documented negative family histories. The mean age of HOC patients at diagnosis was significantly lower (50.2 years) than that of the unselected control population (59 years) (p less than 0.001). Detailed pedigree analysis breaks down the HOC group into (a) site-specific ovarian cancer, 5 cases, 56.4 years mean age; (b) breast-ovarian cancer syndrome, 28 cases, 50.46 years mean age; and (c) Lynch syndrome II (colon/endometrial cancer), 4 cases, mean age 41 years. The age differences were statistically significant (p = 0.050). The most prevalent International Federation of Gynecology and Obstetrics clinical stage at diagnosis of HOC (stage III) was the same as for the control group. Histologically, all (100%) HOC tumors were surface epithelial cancers with predominance of serous papillary type moderate to high grade (89 versus 71% in control, p = 0.07). No other pathologic features appeared to be significant. In conclusion, HOC is a serous papillary tumor and characterized by early age of onset and excess of breast/ovary/colon-endometrial cancers in first-degree relatives of patients with specific HOC syndromes.