Since the application of isoenzyme electrophoresis to the study of Entamoeba histolytica, the prevalence and natural history of asymptomatic intestinal colonization in patients with amebic liver abscess (ALA) has not been addressed. We prospectively evaluated this enteric phase in 50 patients with ALA, using two dosage regimens of metronidazole. The overall prevalence of asymptomatic colonization was 72% (36/50). All these isolates, without exception, proved to express pathogenic zymodemes. Despite a 100% clinical response of the hepatic lesions, failure to eradicate the organism from the bowel occurred in 20 of these 36 subjects. During longitudinal posttreatment surveillance, three carriers returned with second bouts of invasive disease: one with dysentery and two with liver abscesses. Thus, in patients with ALA, there is a high prevalence of intestinal colonization with exclusively pathogenic strains, and treatment with metronidazole frequently results in a continued carrier state. These carriers have a propensity for developing recurrent invasive disease and constitute a public health hazard.