Analysis of multiple biopsies allows a correct diagnosis of inflammatory bowel diseases (IBD) in 66 – 75% of newly diagnosed patients. Additional endoscopic and clinical data allow a final diagnosis in more than 90%.(1, 2) Errors or lack of diagnosis are more frequent (up to 7%) in patients with severe inflammatory activity.(2) The terminology for cases without a definite diagnosis is unclear. “Unclassified” and “uncertain” colitis have been proposed as well as “indeterminate colitis (IC)”.(3, 4) The latter term has become widely used but with a variety of definitions.(5) The common feature of all definitions is that the aetiology and type of colitis can not be identified properly. Pathologists stress the need for colectomy specimens for the diagnosis, while paediatricians, surgeons and gastroenterologists stress the fact that the colitis can not be classified irrespective of the diagnostic tools used. A correct diagnosis is however essential for treatment. For the pathologist who has to provide such a diagnosis, it is important to know the normal histology and the features associated with various forms of colitis and clinical conditions. This paper describes an algorithm for a diagnosis of colitis with a focus on IC.