When considered in the appropriate clinical scenarios, the diagnoses of Crohn's disease or ulcerative colitis are usually straightforward. Most cases can be definitively diagnosed by the typical subacute or chronic history of symptoms, and radiographic, endoscopic, and histologic confirmation in the presence of negative stool studies; newer serologic assays are now available and are of value if the diagnosis remains uncertain. In this paper, we review distinguishing features in the diagnosis of ileitis and the distinction to be made in conclusively diagnosing ulcerative vs. Crohn's colitis.