Segregation analysis of eleven families comprising 2762 individuals indicated compatibility of the data with segregation of a major autosomal dominant gene for hereditary nonpolyposis colorectal cancer. It was estimated that between 71% and 79% of the gene carriers were susceptible and had age of onset that was normally distributed with mean about 47 and standard deviation about 10 years. There was a low frequency of sporadic cases at the older ages, a little less than four percent of nongene carriers being affected by age 80. No significant differences were found between the families exhibiting endometrial cancer (cancer family syndrome) and those not exhibiting endometrial cancer (hereditary site-specific nonpolyposis colonic cancer).