Sex cord stromal tumors are gonadal neoplasms containing Sertoli, granulosa, Leydig, or thecal cells, which originate from cells derived from either the sex cords (Sertoli and granulosa cell tumors) or the specific mesenchymal stroma (Leydig and thecal cell tumors) of the embryonic gonad. Only granulosa and Sertoli cells produce anti-Müllerian hormone (AMH). Our purpose was to investigate whether AMH can be used as a specific marker of human granulosa or Sertoli cell origin in gonadal tumors, to distinguish them from other primary or metastatic neoplasms, using immunohistochemistry. We studied 7 juvenile and 6 adult-type granulosa cell tumors of ovarian localization and 3 extraovarian metastases, 20 other ovarian tumors, 6 testicular Sertoli cell tumors, 2 gonadoblastomas, and 13 extragonadal tumors. Granulosa cell tumors, both juvenile- and adult-type of either ovarian or metastatic localization, showed an heterogeneous pattern of AMH immunoreactivity: Areas containing intensely or weakly AMH-positive cells were intermingled with AMH-negative areas. Although in most cases AMH-positive areas represented a minor proportion of tumor cells, we found a positive reaction in all the cases examined. In testes, although normal prepubertal Sertoli cells were intensely positive, testicular Sertoli cell tumors showed large areas of negative reaction, with few positive cells scattered throughout the tumor. AMH was also reactive in most of the cells of sex-cord origin in gonadoblastomas. No AMH immunoreaction was observed in other gonadal and extragonadal tumors. We conclude that AMH expression is conserved in only a small proportion of tumor cells of granulosa or Sertoli cell origin; however, a positive reaction in a few cells helps to distinguish between granulosa or Sertoli cell tumors or gonadoblastomas and other gonadal tumors of different origin.