Ischemic colitis is the most prevalent form of ischemic vascular compromise of the gastrointestinal tract. Although frequent in the elderly with co-morbidity, numerous pharmacologic agents including diuretics, pseudoephedrine, nonsteroidal anti-inflammatory drugs, oral contraceptives, and cocaine may produce colonic ischemia by mesenteric vasoconstriction. Oral hyperosmotic laxatives are frequently used as cleansing agents in bowel preparation for both radiologic and endoscopic studies. They are regarded as safe and effective agents, but can produce a rapid osmotic-mediated fluid loss, resulting in transient meseneteric hypoperfusion and subsequent ischemic colitis. Here, we describe a case of acute ischemic colitis caused by the oral osmotic laxative magnesium citrate, which was given for bowel preparation before a barium enema in a young healthy patient without underlying disease. (Intest Res 2008;6:135-139)