Loperamide has recently been proposed in the management of infants with severe protracted diarrhea. The purpose of this study was to determine the effect of loperamide (0.5 mg/kg/d) on fecal flora in 19 cases of severe protracted diarrhea. Criteria analysed were: clinical tolerance (vomiting and abdominal distension) and efficacy (number of stools, transit time and Na/k in stools); complete identification and counts of aerobic and strict anaerobic bacteria in fresh stools before and 4 to 8 days after the beginning of loperamide. Parental and/or oral alimentation remained unchanged during the entire study. Clinical resolution of diarrhea was rapid (less than 24 h) in 9 of 14 patients. In 2 cases ileus was observed and resolved when loperamide was discontinued. Although important changes in specific fecal flora counts was noticed for streptococcus D and proteus as compared to 5 controls, no bacterial overgrowth appeared or was worsened during loperamide treatment.