We report a patient with gynecomastia and ectopic production of human chorionic gonadotropin (HCG) by a transitional cell carcinoma of the bladder. In the present case, serum HCG levels and gynecomastia paralleled the clinical course. On admission, the patient was suffering from invasive transitional cell carcinoma (grade 3) of the bladder with metastasis to the left inguinal lymph nodes, together with gynecomastia. The serum HCG level was also elevated. After anticancer chemotherapy, the apparent bladder lesion and gynecomastia disappeared, and the serum HCG level declined to within normal limits. About 2 months after discharge, when the patient suffered from recurrent invasive tumors of the bladder, gynecomastia reappeared and the serum HCG level again became elevated. beta-HCG was demonstrated in biopsy tissue using the immunoperoxidase technique. The presence of beta-HCG was always focally demonstrated and was shown to be localized in the cytoplasm of the tumor cells.