This report concerns evaluation of patient-controlled analgesia (PCA) in the form of two preliminary investigations. In the first study, the patient-controlled analgesia device, which consists of a pump linked to a timer so that patients can activate intravenous administration of morphine sulfate to themselves during the postoperative period, was used in seven morbidly obese patients. The amount of morphine used during the first 36 hours was found to vary between 32 and 185 mg, with a significant difference in drug usage when related to weight as well as to body surface area. In the second study, morbidly obese patients undergoing gastric bypass operations were prospectively randomized into 12 patients who used the PCA device in the postoperative period and 12 patients who were given standard intramuscular dosages of morphine sulfate. An analgesia and sedation scale was then used to compare the two groups. The patients in the PCA group were able to maintain a state of adequate analgesia without sleep with a significantly greater frequency than were those in the intramuscular injection group. On the basis of answers to a questionnaire given to the patient after 60 hours of morphine analgesia, it was apparent that the PCA group was much more satisfied with that form of postoperative analgesia. It would appear that PCA is an efficacious and safe method of providing for postoperative pain relief.