Ischemic colitis appears to be a collection of diseases rather than a single entity. On the one hand, there is the colitis that truly appears to be caused by a lack of blood flow and, on the other hand, there is the disease that is called "ischemic" for lack of a better diagnosis - the colitis that is more "idiopathic" than "ischemic." Four widely held tenets of "ischemic" colitis are wrong: 1) the colon is not particularly sensitive to ischemia; 2) ischemic colitis is rarely preceded by a period of global hypoperfusion; 3) the "watershed areas" are not disproportionately affected; and 4) colonoscopy with biopsy is not specific for the disease. The cause of "ischemic" colitis is unknown. Therefore it is, until proven otherwise, "acute idiopathic colitis.".