Abstract
Genetic and environmental factors are involved in the development of colorectal cancer. The most important prognostic factor is the pathological stage at the time of diagnosis. Therefore it is called for early detection, screening for colorectal cancer, and definition of risk groups. High risk groups are familial polyposis coli, ulcerative colitis, cancer family syndrome, ureterosigmoidostomy, colorectal adenomas, and after resection of colorectal cancer. For these groups a lifelong follow-up and treatment is necessary. But groups with lower risk (Crohn's disease, breast cancer, endometrial cancer, colorectal cancer within the family, gastric polyps, and partial gastrectomy in benign ulcer) need attention too. Colonoscopy with biopsy is one of the most important techniques during follow-up of these patients.