The safety and efficacy of epidural morphine injected into the caudal space for control of postoperative pain following open cardiac surgery in children was studied. Thirty-two children between the ages of 2-12 yr for whom early postoperative tracheal extubation was anticipated were randomly assigned to control and study groups. Study subjects received a caudal injection of preservative free morphine sulfate (0.075 mg/kg) in preservative-free normal saline (5-10 ml) following completion of surgery, but prior to awakening and extubation of the trachea. Supplemental intravenous morphine administration and pain scores were recorded for 24 h. Patients in the study group received significantly less (P less than 0.03) morphine (0.32 mg.kg-1.24 h-1) and had significantly lower pain scores than did patients in the control group (0.71 mg.kg-1.24 h-1). The mean duration of complete analgesia in patients receiving caudally administered morphine was 6 h (range 2-12), but decreased analgesic requirements were noted for the entire 24 h. No respiratory depression was evident by clinical variables or repeated arterial blood gas values. Nausea without vomiting occurred in 4/16 patients in the study group. No patient described pruritus. The authors were unable to evaluate the occurrence of urinary retention because all patients had indwelling urinary catheters. They found caudal epidural morphine to be safe and effective in the treatment of postoperative pain in children following open heart surgery.