Reported gastrointestinal neoplasms in nonhuman primates are reviewed, and the clinical and pathologic features of 11 new cases are described. The 11 monkeys had a total of 12 malignant gastrointestinal neoplasms; one had two primary carcinomas, one in the colon and one in the duodenum. Ten of the 12 tumors were adenocarcinomas: two in the duodenum, one in the jejunum, four in the distal ileum or region of the ileocecal valve and three in the large intestine. The remaining two lesions were a histiocytic lymphosarcoma of the stomach and a poorly differentiated sarcoma of the cecum. The 11 animals included nine Macaca mulatta, one Saguinus oedipus oedipus and one Galago crassicaudatus. All were adults and most were aged. There were six females and five males. Clinical signs included progressive weight loss, a palpable abdominal mass and intermittent diarrhea. Grossly, five of the adenocarcinomas were annular, and constricted the intestinal lumen. Microscopically, the carcinomas generally were well differentiated, and two produced mucin in quantities warranting the modifier "mucinous" adenocarcinoma. All tumors were locally invasive and six of nine monkeys with carcinomas had metastases, with the regional lymph nodes the principal site of involvement.