Forty children undergoing appendicectomy were allocated randomly to receive one of two PCA regimens with morphine. Group B10 received bolus doses of 10 micrograms kg-1 and group B20 received bolus doses of 20 micrograms kg-1. In both groups there was a lockout interval of 5 min and a background infusion of 4 micrograms kg-1 h-1. Group B20 self-administered considerably more morphine (P < 0.01) than group B10. There was no difference between the pain scores of the groups at rest. Group B20 had significantly (P < 0.05) smaller pain scores during movement than group B10 and the latter group suffered significantly (P < 0.01) more hypoxaemic episodes than group B20. There were no differences between the groups in the incidence of vomiting, excess sedation or the amount of time spent asleep at night.