The precise diagnosis of colitis cannot always be established with the available diagnostic tools. The subgroup of patients with an uncertain diagnosis has been classified as "indeterminate colitis" (IC). The definition of "indeterminate," however, has changed over the years. Originally, IC was proposed by pathologists for colectomy specimens, usually from patients operated on for severe colitis, showing overlapping features of ulcerative colitis (UC) and Crohn's disease (CD). Later, the same terminology was used for patients showing no clear clinical, endoscopic, histologic, and other features allowing a diagnosis of either UC or CD. Therefore, it is difficult to compare different studies. An International Organization of Inflammatory Bowel Diseases (IOIBD) working party confirmed 1) the ambiguous nature of the term, and 2) proposes an updated classification for the category of patients with an unclear diagnosis. According to this, the term IBD unclassified (IBDU) is confirmed, as suggested by the Montreal Working Party 2005 for patients with clinically chronic colitis, that clearly have IBD but when definitive features of CD or UC are absent. In resected specimens the term "colitis of uncertain type or etiology" (CUTE) is preferred. It is accepted that most of the time this may have a prefix, such as severe, chronic. The classification of IBD varies when based only on biopsies rather than on a colectomy specimen. The vast majority of these have severe colitis. For those that cannot bear to abandon the highly ambiguous term IC, if it is used at all, this is where it can be used parenthetically.