A prospective randomized controlled trial was conducted in women undergoing emergency and elective caesarean section to determine whether epidural morphine predisposes to postoperative herpes simplex labialis and whether the risk is increased or decreased by previous oral herpes or pruritus. Following cannulation of the epidural space, analgesia for labour and/or caesarean section was induced in two groups of similar age, parity, preoperative status and anaesthetic. According to tables of random numbers, 386 were allocated to receive parenteral morphine and 425 to receive epidural morphine. Postoperative herpes labialis was diagnosed by laboratory culture and microscopy. Patients who received epidural morphine were 11.5 times (95% CI 2.6-49.4) more likely than the parenteral morphine group to suffer herpes labialis. A past history of oral herpes increased the risk of herpes labialis by 8.6 times (95% CI 3.3-22.0). When the influence of past herpes infection and route of morphine were taken into account, no separate association between pruritus and herpes simplex labialis could be demonstrated.