Members of colon cancer-prone nonpolyposis families who had multiple primary malignant tumors were analyzed to determine the frequencies, locations, and stages of their cancers, and the duration of their survival. Colon cancers tended to be more proximal, were in a less advanced stage than in the general population, and in a majority of instances were associated with colonic adenomas. The multiple primary malignant tumors were more common in women, and occurred at a younger age than in the general population. Six or more multiple primary malignant tumors occurred in each of 4 patients. All patients survived for more than 10 yr after the diagnosis of the first cancer. Extracolonic cancers were most frequent in the breast and endometrium.