There are no absolute diagnostic criteria of ulcerative colitis or Crohn's disease, and the diagnosis rests on convergent clinical, laboratory and morphological findings, the most useful being radiological endoscopic and histological data. The diagnosis is often easy, but at the first acute episode all infectious and parasitic causes must be excluded as they may mimic the cryptogenetic diseases. It may be very difficult to distinguish between ulcerative colitis and Crohn's disease when they are restricted to the colon and rectum. In this respect, diagnostic score systems can be very helpful.