Patient-controlled analgesia (PCA) with morphine is commonly used to provide acute postoperative pain control after major surgery. The fentanyl hydrochloride patient-controlled transdermal system eliminates the need for venous access and complicated programming of pumps. To assess the efficacy and safety of an investigational patient-controlled iontophoretic transdermal system using fentanyl hydrochloride compared with a standard intravenous morphine patient-controlled pump. Prospective randomized controlled parallel-group trial conducted between September 2000 and March 2001 at 33 North American hospitals, enrolling 636 adult patients who had just undergone major surgery. In surgical recovery rooms, patients were randomly assigned to intravenous morphine (1-mg bolus every 5 minutes; maximum of 10 mg/h) by a patient-controlled analgesia pump (n = 320) or iontophoretic fentanyl hydrochloride (40- microg infusion over 10 minutes) by a patient-controlled transdermal system (n = 316). Supplemental analgesia (morphine or fentanyl intravenous boluses) was administered as needed before and for the first 3 hours after activation of the PCA treatments. Patients then used the PCA treatments without additional analgesics for up to 72 hours. The primary efficacy variable was patient global assessment of the method of pain control during the first 24 hours. Additional efficacy measures were the proportion of patients discontinuing the study because of inadequate analgesia for any reason, patient-reported pain intensity scores on a 100-mm visual analog scale (VAS), and patient global assessments at 48 and 72 hours. Adverse effects were also recorded. Ratings of good or excellent after 24 hours of treatment for the method of pain control were given by 73.7% of patients (233/316) who used transdermal fentanyl PCA and 76.9% of patients (246/320) who used intravenous morphine PCA; treatment difference was -3.2% (95% confidence interval, -9.9% to 3.5%; P =.36). Early patient discontinuations (25.9% fentanyl vs 25.0% morphine; P =.78) and last pain intensity scores (32.7 fentanyl vs 31.1 morphine on the VAS; P =.45) were not different between the 2 treatments. With continued treatment for up to 48 or 72 hours, more than 80% of patient assessments in each treatment group were good or excellent. The incidence of opioid-related adverse events was similar between the groups. An investigational PCA transdermal system using iontophoresis to deliver fentanyl provided postsurgical pain control equivalent to that of a standard intravenous morphine regimen delivered by a PCA pump.