Several studies have found an increased risk of colorectal cancer associated with a family history of colorectal cancer. Some studies, although not all, have also suggested that family history of colorectal cancer may be a risk factor for adenomatous polyps. Hereditary nonpolyposis colorectal cancer is a known genetic syndrome predisposing to colorectal cancer. The hypothesis of this paper is that the preponderance of the genetic or familial risk for colorectal cancer in the United States is mediated by hereditary nonpolyposis colorectal cancer. To test this hypothesis, I have incorporated what is known about hereditary nonpolyposis colorectal cancer into a genetic model that generates probabilities of family clustering of colorectal cancer. Using this model, which assumes that all familial risk for colorectal cancer is due to hereditary nonpolyposis colorectal cancer, the expected relative risks for colorectal cancer (and adenomas) associated with given types of family histories were calculated. The relative risks predicted by the model fairly closely matched the results found in the literature, especially those reported from a large cohort study. As observed in several studies, the model predicts that relative risks decrease sharply with age. In contrast to the elevated risk for colorectal cancer, the model predicts no elevated risk for adenomas associated with family history of colorectal cancer.