The clinical and pathologic features of 71 endodermal sinus tumors of the ovary were studied in an effort to delineate the histogenesis and biologic behavior of this neoplasm and to evaluate the efficacy of different forms of treatment. Alpha-fetoprotein (AFP) was identified in hyaline droplets, cell cytoplasm, and intercellular spaces of all 15 tumors examined by an immunoperoxidase technique; this supports the view that the neoplasm simulates yolk sac endoderm. There were only nine survivors among 65 patients on whom follow-up information was available; the actuarial survival was 13% at 3 years. Of the neoplasms that recurred, 93% did so within 1 year, and of those patients who died, 93% did so within 2 years. The size and stage of the tumor had prognostic significance, but the patient's age, the mitotic activity, and histologic pattern did not. Although 71% of the patients had Stage I tumors at the time of diagnosis, subclinical metastasis was present in 84% of Stage I patients. Triple chemotherapy (vincristine, actinomycin D, and cyclophosphamide (VAC)) employed after unilateral salpingo-oophorectomy in four patients with Stage I tumors resultivors among 12 Stage I patients treated with combined surgery and radiation. The finding of AFP in all tumors in which this was evaluated suggests that serum radioimmunoassay might be useful to monitor response to therapy.