UNLABELLED Studies in animals and volunteers have suggested the development of acute tolerance to opioid analgesics. In this article, we present data from patients who regulated their own target-controlled infusions of alfentanil and remifentanil to provide analgesia in the immediate postoperative period. Fifty-one patients received alfentanil for 24 h after cardiac surgery, and 30 patients received remifentanil for 6 h after orthopedic surgery. Satisfactory analgesia, defined as a rating of < or =3 on an 11-point visual analog scale, was obtained by patients after each type of surgery. The target concentrations of the opioids required to produce postoperative analgesia and the cumulative opioid doses administered over the course of the clinical observation suggest there was no tolerance to the analgesic effects of the opioids. The requirements for both analgesic drugs in individual patients had a large variation (>200%). We conclude that our results may indicate an absence of tolerance to opioids in postoperative analgesia. Nonetheless, our data show that the postoperative requirement for these rapidly acting drugs is qualitatively similar to that for other opioids in that dosage escalation does not occur. IMPLICATIONS The development of acute tolerance to opioid analgesics has been suggested based on experimental studies in animals and volunteers. Our report from patients who self-controlled their analgesic requirements by using target-controlled infusions of alfentanil and remifentanil for postoperative analgesia provides no evidence of tolerance to opioids.