When evaluating patients suspected to have inflammatory bowel disease, we generally focus on the important differential features which distinguish ulcerative colitis from Crohn's disease. A distinction between them is possible in 80-90% of cases when accepted clinical, radiographic, and pathologic criteria are applied. The remaining idiopathic cases are usually classified as colitis, indeterminate type. In this presentation, major attention will also be directed to a variety of other disorders which may masquerade as idiopathic inflammatory bowel disease. The pathologic features to be described which serve to identify these "impostors" will emphasize appearances in biopsy material.