Spermatocytic seminoma is a rare testicular neoplasm that occurs only in adults (range, 25 to 87 years; mean, 54). It has no ovarian homologue and is found only in descended testes. It is not associated with other types of germ cell neoplasia. Orchiectomy is curative in virtually all cases; of more than 200 known cases, only one has metastasized. In a dozen cases sarcomas have arisen in the testes in association with spermatocytic seminoma; most of these patients have died of metastatic sarcoma after short intervals. The gross appearance is characteristically edematous or gelatinous and more than half the tumors have been more than 5 cm in diameter. Microscopically, they are composed of sheets of small, medium, and large cells with spherical nuclei. The chromatin is dense in the small cells and filamentous in the medium and large ones. The cytoplasm is eosinophilic to amphophilic and lacks glycogen. It is not associated with intratubular germ cell neoplasia of the undifferentiated type. Analyses of DNA content have failed to show haploid populations and the lectin binding does not show maturation toward spermatocytes. Differing from seminoma in essentially all characteristics, it is not a variant of seminoma.