The Cancer and Steroid Hormone Study, a multicenter, population-based, case-control study of ovarian, breast, and endometrial cancer in women 20 to 54 years of age, permitted the diagnoses of contributing pathologists to be compared with those of a panel of three gynecologic pathologists. A diagnosis of ovarian cancer was made by contributing pathologists on 477 subjects. Agreement between the two groups of pathologists was 97% for primary epithelial ovarian cancer and 89% for primary nonepithelial ovarian malignancies. Agreement on diagnosis of major cellular subtypes of ovarian malignancy ranged between 73% for endometrioid cancer and 100% for clear cell carcinomas. We conclude that the diagnosis of pathologic features of primary ovarian cancer is highly predictable. Nonetheless, diagnosis by histologic type varies sufficiently that a review process should be considered for clinical or investigative decisions involving specific histologic diagnoses of ovarian cancer.