Colon cancer occurring in patients with Lynch Syndrome and in Inflammatory Bowel Disease (IBD) share many features. There is some evidence to support the assumption that multiple genetic factors play an important role in the pathogenesis of idiopathic IBD and Lynch Syndrome. In our previous study, providing detailed medical, genetic and pathologic findings on 202 hereditary non polyposis colorectal cancer (HNPCC) relatives we found in the colonic mucosa features indicating an IBD though all the screened subjects of the family denied symptoms of IBD. Some studies have reported that the rate of undetected IBD ranges from 27 to 38%. Finally, a member of this family, considered not at risk for cancer by genetic analysis results, developed a clinically manifested IBD. The morphological aspects of the disease were not discussed in our previous study. It is possible that many members of this family inherit a major gene giving liability to the disease and are carriers of a subclinical form of IBD with a minimal morphological marker which becomes manifest in some members when other factors intervene. A possible genetic model linking the two diseases can be suggested: IBD needs two major genes for susceptibility (s) and clinical development (D). Both can be present in IBD and Lynch Syndrome, but in the latter a third gene plays a suppressor role on the development gene (D). In conclusion, we hypothesize that the IBD developing gene may be considered as protective against HNPCC, and this condition may result in a selective genetic advantage.