Amebiasis remains an extremely important consideration in the differential diagnosis of diarrhea, especially when there is associated bleeding. Amebic colitis may be, and was, in all 8 patients reported here, indistinguishable clinically and sigmoidoscopically from ulcerative, transmural, or granulomatous colitis. Amebiasis was diagnosed promptly and accurately in patients sigmoidoscoped without preparation or interfering substances and from whom fresh exudate was obtained at the time of sigmoidoscopy and examined immediately for the presence of motile hematophagous trophozoites of Entamoeba histolytica. Amebae were seen in only five of 12 pretreatment colonic mucosal biopsies. A diffuse inflammation, compatible with mucosal abnormalities seen in ulcerative or granulomatous colitis, was present in all biopsies, and was the only histopathological abnormality in seven. Amebic serology (indirect hemagglutination) was positive in all patients and prompted a search for amebae in 2 patients who had been misdiagnosed as having probable granulomatous colitis. All of the patients treated with metronidazole experienced clinical cures. Parasitological failures were seen, however. It is imperative that appropriate studies to establish or exclude the diagnosis of amebiasis be carried out in all patients who present with a clinical and sigmoidoscopic picture of colitis, and that patients treated with metronidazole for amebiasis have adequate clinical and parasitological follow-up.