Undifferentiated carcinoma of the ovary. Thirty-five cases of ovarian carcinomas, which had as the predominant histologic component solid areas of epithelial cells without differentiation into mullerian carcinomas, were reviewed. The patients' ages ranged from 39 to 72 years (mean age, 54 years). Two patients had clinical stage I disease, one had stage II, 26 had stage III, and six had stage IV. Microscopically, the malignant cells formed large groups or sheets with desmoplastic stroma around them. Foci of papillary serous carcinoma, unclassified adenocarcinoma, or transitional cell carcinoma were seen in 26 tumors, foci of necrosis were seen in 30 tumors, and vascular invasion was seen in seven tumors. Six of 13 carcinomas tested expressed CA125 reactivity, and 12 of 13 carcinomas reacted to B72.3 monoclonal antibody. The primary tumors were treated by aggressive surgical reduction in 32 patients and by multiple biopsy procedures in three patients. After the first operation, 30 patients had residual disease, smaller than 2 cm in five patients and larger than 2 cm in 23 patients. After surgery, 33 patients received chemotherapy; three of these 33 also received radiotherapy. One patient was treated with postsurgical radiotherapy only, and one patient refused further treatment. Thirty-four patients (97%) died of disease between 8 and 108 months (mean, 27 months) after initial surgery, 29 patients died in less than 32 months. Four patients (11%) survived more than 5 years: two patients with stage I disease who died at 82 and 102 months, one patient with stage II who died at 72 months, and one patient with stage III who has no evidence of disease after 116 months. Five-year survival of patients with undifferentiated ovarian carcinoma is worse than the reported survival of patients with serous carcinoma or ovarian carcinoma with a pattern resembling transitional cell carcinoma. The distinction between these three carcinomas that have solid areas carries prognostic significance. 