This patient most likely has metastasis from a choriocarcinoma originating in his testicles.13-Human chorionic gonadotropin (j3-hCG) is released from the syncytiotrophoblast poion of the tumor. The gynecomastia most likely results from stimulation of breast tissue by the 3-hCG, which is a luteinizing hormone (LH) analog. Leukemias or multiple myeloma originating from the bone marrow are not associated with gynecomastia. Kidney tumors, especially renal adenocarcinoma, can ectopically secrete gonadotropins; however, renal tumors do not usually occur in patients this young. Islet cell tumors of the pancreas do not ectopically secrete 13-hCG or gonadotropins. Most anterior pituitary tumors are benign. Primary gonadotropinsecreting tumors are rare.