The cooperative therapy study MAKEI 83/86 included an examination of the prognostic value of the AFP in children and adolescents with extracranial non-testicular yolk sac tumors. The serum AFP values of 72 protocol- and follow-up-patients were documented at diagnosis and up to the ninth month of treatment. 32 of these patients had saccrococcygeal tumors, 27 had tumors of the ovary and 13 suffered from extragonadal germ cell-tumors. 4 children showed progressive disease under initial chemotherapy and 1 patient died of therapy, 10 of 72 patients relapsed. The AFP measurements were plotted on semilogarithmic charts. They were compared to the measurements of healthy children up to the age of 1 year. According to the development of the patients' AFP values compared to the reference curves the following classifications could be made: 1. Patients with a normal AFP-decrease id est 50% in less than or equal to 6 days during the 1st month of therapy: 48/72 patients 2. Patients with slow AFP-decrease: 17/72 patients 3. Patients with transient AFP-decrease: 5/72 patients 4. Patients with no AFP-decrease: 2/72 patients According to Kaplan-Meier life table analysis, patients with a normal AFP-decrease had an event-free survival of 89% +/- 4%, whereas all other patients showed an event-free survival of 63% +/- 10% (p less than 0.05). Regarding primary therapy id est tumor resection or preoperative chemotherapy an equal distribution of the patients among those with a normal and slow AFP-decrease was observed.(ABSTRACT TRUNCATED AT 250 WORDS)