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The age-related incidence of malignant neoplasia was surveyed from a total of 301 necropsy cases of rhesus monkeys ranging in age from 13-37 years performed in the Pathology Service Unit of the Wisconsin Regional Primate Research Center during the past 15 years. All our aged monkeys lived in indoor cages and were fed with monkey chow and supplemental fruits during the past decades. In this survey, we found a total of 51 malignant neoplasms, and among them 25 cases were colon cancer. The incidence of colon cancer increased with advancing age: 3.2% at 13-19 years, 9.2% at 20-25 years, 13.5% at 26-29 years, and 20.7% at 30-37 years. Most cancers were located in the cecum and transverse regions with a unicentric origin. Two multicentric cases were associated with chronic hypertrophic colitis. Precancerous polypous lesions were not found in all cases. Histologically, all cases were mucinous adenocarcinoma and had local invasion to the muscular wall. Metastasis to the mesenteric lymph nodes was found in only two cases. As in humans, colon cancer is a common outcome of aging in nonhuman primates.