BACKGROUND: Postoperative vomiting is a troublesome problem in pediatric patients undergoing strabismus surgery. We compared the effect of clonidine and midazolam to prevent vomiting in a randomized, single-blind, placebo-controlled study of 60 healthy children undergoing surgical correction of strabismus. METHODS: The children, aged 3~15 years, were randomly allocated into three groups. The children in group 1 received clonidine 4 microgram/kg per oral about 90 min before induction of anaesthesia. Those in group 2 received midazolam 75 microgram/kg intravenously after induction before surgery. Those in group 3 received a placebo. Anaesthesia consisted of enflurane, nitrous oxide in oxygen, vecuroniun and thiopental. RESULTS: There was no difference among the groups with respect to age, gender, weight, duration of anaesthesia and recovery room time. The data showed that clonidine 4 microgram/kg per oral before induction of anaesthesia was significantly effective in preventing vomiting (P<0.017). But there was no significant difference between the midazolam group and the placebo group. CONCLUSION: We recommend the prophylactic administration of clonidine 4 microgram/kg per oral to children 3~15 years old because it decreases postoperative vomiting. It also provides preoperative sedation, postoperative analagesia, perioperative hemodynamic stability and a reduction in the volatile anesthetic requirement.