Familial risk of colon cancer is common. The rare syndromes of FAP and HNPCC account for a small fraction of familial cases. A working knowledge of these syndromes is important, however, because appropriate management can prevent most cases of colon cancer in families with these conditions. Furthermore, the genes that give rise to FAP and HNPCC have been identified, and genetic testing to find gene carriers in affected families is now commercially available. In addition to the syndromes, colon cancer commonly clusters in families. The risk of colon cancer is two to threefold increased when a first-degree relative is affected with this malignancy. The risk is further increased if multiple first-degree relatives have colon cancer or if a diagnosis in a relative is made at an age of 50 years or younger. The genetic causes of this more commonly observed of familial clustering have not been clarified, but screening based on the degree of familial risk has been suggested.