Patients with inflammatory bowel diseases háve an increased risk of developing cancer. Duration of disease, extent of inflammation, presence of primary sclerosing cholangoitis and onset of disease in childhood are among the most important factors contributing to tumour genesis. However, this risk can be lowered with periodical colonoscopy, good patient/physician collaboration, and mesalazine or folie acid treatment. It is very important to look out for preneoplastic changes (mainly dysplasia) and to properly evaluate them. It is advisable to establish a reasonable preventivě program for the early detection of preneoplastic changes or tumours in order to deerease the mortality from this serious disease.

        Key words: inflammatory bowel diseases, dysplasia, colorectal cancer, colonoscopy