Objective To compare the febrile morbidity rate after elective abdominal hysterectomy between patients given single dose vs 24-hour-dose intravenous ampicillin prophylaxis.Study design Randomized controlled trial.Setting Department of Obstetrics and Gynecology, Faculty of Medicine, Prince of Songkla University.Subjects Eighty-seven patients who underwent elective abdominal hysterectomy atSongklanagarind hospital from March to December 2002.Intervention The subjects were randomly allocated to receive either single dose or 24-hour-dose intravenous ampicillin prophylaxis.Main outcome measures Febrile morbidity rate in the two groups.Results The characteristics and risk factors for postoperative infection among the two groups were not statistically significantly different. There was no statistical difference in febrile morbidity between the two groups. The febrile morbidity rate was 5/43 (11.6%) and 3/44 (6.8%) in the single dose and 24-hour-dose, respectively.Conclusion Single dose and 24-hour-dose of intravenous ampicillin prophylaxis showed no significant difference in preventing febrile morbidity after elective abdominal hysterectomy.