Among the paraneoplastic endocrine syndromes, gynecomastia induced by an ectopic production of human chorionic gonadotrophin remains infrequent. The authors report the case of a fifty-year-old man presenting with a bilateral gynecomastia which had been evolving over three months. The etiological investigation was rapidly guided by a macroscopic hematuria. Intravenous nephro-urotomographies visualized a filling defect in the bladder, and a grade III stage A urothelial carcinoma was completely resected cystoscopically, leading to progressive disappearance of the gynecomastia. Hormonal investigation revealed an ectopic production of HCG, inducing hyperestrogenia.