AbstractWe describe a boy who developed precocious puberty resulting from chorionic gonadotropin produced by a mediastinal germ cell tumor. Following tumor removal he began spontaneous precocious sexual development which was treated and then arrested spontaneously. Investigation of this arrested puberty established that he had Klinefelter syndrome (KS) mosaicism. He represents the first instance of KS mosaicism reported with a mediastinal germ cell tumor, a neoplasm commonly reported in males with a 47,XXY karyotype. We recommend that all males with KS and early sexual development or with “normal” testicular growth be screened with measurement of germ cell tumor markers including β‐subunit of human chorionic gonadotropin and α‐fetoprotein. © 1995 Wiley