From 1982 to 1989 positive alpha-fetoprotein (AFP) levels were detected after surgery in 3 patients with pure immature teratoma, 7 patients with mixed germ cell tumors and 16 patients with pure endodermal sinus tumor (EST) of the ovary. In the three patients with pure immature teratoma, AFP serum levels became spontaneously undetectable in one month without chemotherapy. The marker was thereafter negative, despite the persistence of tumor in 2 patients, and a relapse in the third one. The serum levels of AFP did not correlate with the stage of EST and mixed germ cell tumors. The monitoring of AFP serum levels during treatment correlated with response to chemotherapy: of the 16 patients with pure EST, 14 achieved a serological complete response and did not undergo second look surgery; two patients had a serological partial response. One of them reached a serological complete response after further chemotherapy. The other patient with liver extragonadic tumor underwent liver lobectomy with negative histology. Among 7 patients with mixed germ cell tumor, 6 achieved a serological and pathologic complete response. The 7th patient had a serological complete response, but microscopic residual tumor at second look laparotomy. The decrement curve of AFP was not predictive for recurrence. However, AFP monitoring was useful for an early diagnosis of relapse in 4 patients. AFP is a reliable marker for the diagnosis, treatment evaluation and follow-up of patients with pure EST and a useful marker in the clinical management of mixed germ cell tumors of the ovary.