We describe a patient with advanced bladder carcinoma, in which ectopic synthesis and secretion of human chorionic gonadotropinlike immunoreactivity (ihCG) was demonstrated. Concanavalin A-sepharose affinity chromatography of serum revealed that the ihCG differed from placental human chorionic gonadotropin in its carbohydrate moiety. Treatment of the patient with a cisplatin-containing regimen did not result in regression of the tumor. Since immunohistochemistry revealed ihCG in five of 13 additional cases of bladder carcinoma, it is concluded that ectopic production of ihCG by bladder carcinomas is probably not rare.