OBJECTIVE To compare the clinical efficacy of theoretically equipotent doses of fentanyl and sufentanil, each in combination with bupivacaine, for patient-controlled epidural analgesia (PCEA) for labor. METHODS Single-blinded, randomized study. METHODS University hospital. METHODS 48 ASA physical status I and II term parturients in active labor, with cervical dilatation equal to or less than 5 cm, and requesting epidural analgesia. METHODS Patients received a loading dose of 10 mL of 0.125% bupivacaine with either 30 microg of fentanyl or 5 microg of sufentanil. PCEA was maintained with 0.0625% bupivacaine with either fentanyl two microg/mL or sufentanil 0.35 microg/mL. The PCEA settings were: bolus 5 mL, lockout 10 minutes, infusion 10 mL/hr, and maximum dose 40 mL/hr. Standardized rescue doses of bupivacaine were administered as necessary. METHODS The bupivacaine requirement in mg/hr was calculated from the time of initiation of the epidural until the patient had had 4 to 6 hours of PCEA. The hourly pain score, sensory and motor block, and side effects were documented. Overall patient satisfaction was assessed at the end of the study. RESULTS The mean (standard deviation) bupivacaine requirement was 12.4 mg/hr (3.2) and 11.0 mg/hr (2.4) for the fentanyl and sufentanil groups, respectively (P = 0.08). There was evidence of higher maternal satisfaction (P = 0.01), and weak evidence of lower pain scores (P = 0.10) in the sufentanil group. The side effects were similar in both groups. CONCLUSIONS At the assumed equipotent sufentanil to fentanyl ratio of 6:1, there is some evidence that sufentanil is clinically superior to fentanyl as an adjunct to bupivacaine in labor epidurals, although the advantages are subtle.