The incidence of endodermal sinus tumor (EST) or yolk sac tumor (YST) elements has been studied in two series of testicular germ-cell neoplasms in adults. One series, consisting of 200 germ-cell neoplasms seen from 1053 through 1968, was studied retrospectively, and the other, consisting of 147 cases seen from May 1974 through February 1979, was studied prospectively. Excluding the cases of pure seminoma, EST(YST) elements were found in 21 (28.7%) of 73 cases in the retrospective series and in 27 (44.4%) of 61 cases in the prospective series. The EST(YST) elements were in all cases admixed with other neoplastic germ-cell elements and varied from microscopic foci to being the predominant element within a tumor. The EST(YST) elements were histologically similar to infantile EST(YST) and EST(YST) in other locations. Serum alphafetoprotein (AFP) was determined in the majority of patients in the prospective series, and there was good correlation between the presence of EST(YST) elements within the tumor and elevated levels of AFP. The results of the present study indicate that EST(YST) elements occur quite frequently in testicular germ-cell neoplasms in adults and provide an explanation for the raised levels of serum AFP found in many adults with testicular germ-cell tumors. The results emphasize the importance of a thorough and careful pathologic examination of testicular germ-cell tumors, and the value of AFP as a tumor marker in patients with EST(YST). The results also lend further support to the view that EST(YST) elements found in testicular germ-cell tumors in adults are homologous with infantile EST(YST) and that EST(YST) is a distinctive and specific type of germ-cell neoplasm and should be included as such in the classification of testicular tumors.