The age and family history of colorectal cancer are the main risk factors for colorectal cancer. Taking into account the family history of colorectal cancer, the first degree relatives of a patient have a risk that increases to 2 or 3 as compared to general population. The genetic basis for colorectal cancer was recently investigated by complex segregation analysis. They favoured a dominant gene with a gene frequency of 6 :1000. These analyses represent the first step in order to localize the susceptibility genes that are responsible for the familial aggregation of colorectal cancer.