Entamoeba histolytica is a protozoan that is endemic in various parts of the world, including some areas of the United States. It may live in the large bowel in its cyst form without harming the host (commensalism) or, for as yet poorly understood reasons, invade the tissues as a trophozoite producing invasive amebiasis of the colon. In a review of over 3,000 cases of invasive amebiasis, the clinico-pathologic forms of the disease were: ulcerative rectocolitis (95%), typhloappendicitis (3%), ameboma (1.5%), and fulminating colitis and toxic megacolon (0.5%). Different radiographic patterns are seen in each clinical form with varying degrees of specificity. It is vitally important that this disease be included in the differential diagnosis of large bowel pathology even in nonendemic areas. Several referral patients who have received inappropriate therapy for inflammatory bowel disease with near disastrous results are seen at one of our institutions (Loyola) each year.