Yolk sac tumor (YST) or endodermal sinus tumor is the most common malignant germ cell tumor of the testis in children, and is the second most frequent malignant germ cell tumor of the ovary, being exceeded in frequency by dysgerminoma (seminoma). The incidence of malignant tumors of the testis was 1.0 cases per million U.S. white children less than 15 years of age per year. The corresponding figure for U.S. black children was 1.7 285. There were no published figures of the incidence rates of specific histologic types of germ cell tumors among U.S. children. The incidence rate of yolk sac tumor of the testis was relatively constant in those populations for which incidence figures have been reported. The incidence rate of yolk sac tumor of the ovary was similar to that for the testis (Table 1) 22,23,252.