Forty-four patients with clear cell adenocarcinoma of the ovary diagnosed between 1944 and 1981 were compared with a matched cohort of 55 patients with the most common epithelial malignant lesion, serous adenocarcinoma, in terms of their presentation and clinical course. None were lost to follow-up. Median follow-up was 9 years. Fifty percent of clear cell patients presented in Stage I versus 31% of serous patients. Patients with clear cell carcinoma presented more often with pelvic masses (84% vs 65%) and had larger (diameter greater than 10 cm) primary tumors (73% vs 29%). Forty-nine percent of clear cell patients were nulligravid compared with 24% of serous patients and endometriosis was strikingly more common in clear cell patients (58% vs 12%). When compared stage for stage, clear cell tumors were uniformly associated with poorer 5-year survival rates with an overall rate of 34%. In patients with recurrent disease, lymph node involvement was much more common in patients with clear cell carcinoma (40% vs 7%). Parenchymal organ involvement was also more common in the clear cell group (40% vs 13%). Ovarian clear cell adenocarcinoma has distinctly different clinical behavior compared to serous carcinoma and should be regarded as an aggressive epithelial histologic type.