The presence of enteroaggregative Escherichia coli (EAggEC) in stool has been strongly associated with persistent diarrhea. No treatment trials have been done to demonstrate that clearance of EAggEc results in an improvement of diarrheal symptoms. Twenty-four adults infected with the human immunodeficiency virus (HIV) with diarrhea and EAggEC were randomized to a double-blind placebo-control cross-over treatment trial (ciprofloxacin 500 mg orally twice daily for 7 days vs. placebo). After treatment with ciprofloxacin, the subjects had significantly fewer (50%) stools per day (from 5.0+/-2. 9 to 2.4+/-1.9). Intestinal symptoms decreased by 42% after active treatment. EAggEc were eradicated from stool of all participants after active treatment. These data strengthen the link between the presence of the EAggEc in stool and their role in the pathogenesis of diarrheal disease. It is likely that EAggEc are a treatable cause of diarrheal disease in some persons with HIV and no other apparent enteric pathogen.