Patient with HNPCC syndrome confirmed by a mutation (MLH1, MSH2, MHS1) are involved in the study. Patient have 2 colonoscopy back to back. The second coloscopy is associated to chromoscopy with carmin indigo. Endoscopist are randomised for the colonoscopy with chromoscopy and are un-awarded of the result of the first colonoscopy. Histopathology of the polyp are noted. The follow up were 1 month.