Although the presence of intratubular trophoblast (ITT) has been reported, its frequency and distribution in association with germ cell tumors (GCT) have not been investigated. Beta human chorionic gonadotropin (hCG) is a sensitive immunohistochemical marker of syncytiotrophoblast. We therefore wished to investigate whether intratubular trophoblastic elements could be identified adjacent to invasive tumors using immunohistochemistry against hCG. Seventy-five GCTs were examined. Immunochemistry was performed for hCG. Both invasive tumor and seminiferous tubules were examined for positive staining. The seminomas showed ITT in five of 29 cases. All these cases had trophoblastic cells as part of the invasive tumor. Only one of 36 cases of nonseminoma and one of nine of the mixed GCTs (11%) showed ITT. Again, all of the positive cases had hCG-positive trophoblastic cells within the invasive tumor. ITT can be identified adjacent to GCTs in a significant number of cases. We suggest that this is a genuine in situ lesion, associated with seminomas with syncytiotrophoblastic cells. Differentiation toward trophoblastic elements in GCTs may occur at an earlier stage of their pathogenesis than has been previously recognized.