Hereditary non polypous colon cancer (HNPCC) is inherited in a dominant fashion, and accounts for 29-39% of all colon and rectum carcinomas occurring in people under 50 years of age. We present a case of a young male with a short history of fluctuating diarrhoea and bleeding per rectum. Family records showed a high incidence of colon and rectum cancer. Coloscopy showed a colon tumour that was treated by total colectomy. To obtain the correct diagnosis a thorough family history is crucial. Having found a sick individual in a family it is now possible to identify genotype-carriers among family members.