We describe a 16-year-old boy with Leydig cell tumor who initially presented bilateral gynecomastia with increased estradiol concentrations, decreased testosterone concentrations and normal gonadotropin levels. Testicular ultrasonography showed a tumor in the left testicle, and orchidectomy was performed. Histopathological analysis revealed a Leydig cell tumor. Two months after surgery, the gynecomastia diminished gradually and estrogen levels returned to normal. No recurrences have occurred during a 2-year follow-up.