Fourteen cases of yolk sac carcinoma, 10 occurring in gonadal, and four in extragonadal sites, seen at the Indiana University Hospitals from 1949 to 1974, were analyzed with respect to pathologic features, laboratory findings, and clinical course. Their histologic appearance was similar regardless of the site of origin. Two basic histologic types were observed--the more common endodermal sinus pattern and the rare polyvesicular vitelline form. The prognosis is unfavorable, but three of our cases exhibited objective responses to chemotherapy. In our small series, the better prognosis of testicular yolk sac carcinomas in children found by some authors was not evident. Four of the 6 patients with yolk sac carcinoma in which serum alpha-fetoprotein determinations were performed showed positive results. Three of these cases had residual or metastatic disease clinically. The demonstration of alpha-fetoprotein in the serum of patients with yolk sac carcinoma lends further support to the yolk sac origin of these tumors and could also prove to be of prognostic value by indicating the presence of residual or recurrent disease.