Summary: A diagnosis of Crohn's disease (CD) and ulcerative colitis (UC) is based on a combination of clinical, histologic, endoscopic, and radiologic data. The distinction between UC and CD can be difficult because of the lack of a differentiating single gold standard. Indeterminate colitis (IC) was introduced by pathologists for the diagnosis of surgical colectomy specimens showing an overlap between the features of UC and CD. The diagnosis of IC was based on macroscopic and microscopic features. The term indeterminate colitis is in recent years more widely applied to include all cases with endoscopic, radiographic, and histologic evidence of chronic inflammatory bowel disease confined to the colon, but without fulfilment of diagnostic criteria for UC and CD. As for UC and CD, the diagnosis of IC has therefore become a clinicopathologic diagnosis. IC is generally considered to be a temporary diagnosis. The clinical characteristics of patients with IC are, however, somewhat different from the characteristics of those with UC. Furthermore, serologic markers such as perinuclear antineutrophil cytoplasmic antibody and anti-Saccharomyces cerevisiae, which are strongly linked with UC and CD, are both negative in a subset of patients with IC. Therefore, the possibility that IC could be a separate entity must be investigated.