We report a case of mixed tumor of the stomach with a double histological population: carcinoid and adenocarcinoma. The carcinoid component presented an usual morphological pattern with "atypical" elements, and without argyrophilic cells; the carcinoid cells were exclusively marked by an anti-serotonin serum. The other component was a poorly differentiated carcinoma, with muco secretion, which was not marked by and anti-carcino-embryonic antigen serum. The limit between the two components was sharp. The association of mucin in the carcinoma and serotonin in the carcinoid component was an argument in favour of the diagnosis of mixed tumor carcinoid-adenocarcinoma (composite tumor) of the stomach. The evolution after operation leaded to a local recurrence and to the death 10 months later, in spite of chemotherapy. The existence of composite tumors raises nosological, histogenetical and diagnostic problems, which are discussed in this work. The prognosis of these tumors is unknown and therapeutic possibilities seem to be scanty.