Early age onset and proximal colonic location are two specific characteristics of colon cancer which have been used clinically to assess the risk that an individual case is of familial rather than sporadic origin. This practice derives from the observation that these characteristics are typical of the rare, nonpolyposis inherited colorectal cancer syndromes. This study examines these two characteristics in cases of common colon cancer to determine whether they actually distinguish individuals at increased risk for familial colorectal cancer. Familial clusters of colon cancer in the Utah Population Data Base were examined. Common colon cancers were found to cluster excessively in families; however, the measure of familial clustering for distal colonic cases was increased to the same degree as proximal colonic cases. Early age onset was likewise not a distinguishing factor of familial cases. These results suggest that factors other than those that predispose to the rare syndromes are important in determining familial risk for common colon cancers, and that the absence of these two clinical features should not suggest the absence of familial risk of colorectal cancer.