The authors compared the duration of analgesia and the frequency of side effects of three doses of caudal epidural morphine in children aged 1.2-7.9 yr. Caudal catheters were inserted in 32 children, randomly assigned to receive 0.033 mg.kg-1, 0.067 mg.kg-1, or 0.10 mg.kg-1 of preservative-free morphine for analgesia after major surgical procedures below the diaphragm. The first dose of caudal morphine was mixed with 0.25 ml.kg-1 of 1% lidocaine to confirm correct caudal catheter placement. By assessment of periodic pain scores and the time intervals between administration of caudal morphine and the recurrence of pain, the authors found that the mean (+/- SD) duration of analgesia was significantly longer after 0.10 mg.kg-1 (13.3 +/- 4.7 h) than after either 0.033 mg.kg-1 or 0.067 mg.kg-1 (10.0 +/- 3.3 and 10.4 +/- 4.2 h, respectively) (P less than 0.02). The frequency of vomiting, pruritus, and urinary retention was similar in each group. Vomiting was less common in patients who had nasogastric drainage than in patients who were fed soon after surgery (P less than 0.05). Delayed respiratory depression occurred in one child after 0.10 mg.kg-1 of caudal morphine. Caudal morphine, 0.033-0.10 mg.kg-1, provided prolonged analgesia in children. The authors recommend 0.033 mg.kg-1 of caudal morphine as an initial dose for children.