Background: Opioids added to local anesthetics for peripheral nerve blocks may intensify analgesia and prolong analgesic and sensorial block duration. These agents may also cause potentiation and prolongation of motor block. Objective: This study compared the postoperative effects of 30 mL of 0.25% bupivacaine +50 mcg fentanyl and 30 mL of 0.25% bupivacaine + 100 mcg fentanyl solutions for the ultrasound-guided infraclavicular block in patients undergoing elbow and forearm surgery. Methods: In this randomized double-blind study, thirty-six patients with risk of ASA class I-III were randomly allocated into 2 randomized groups. Ultrasound-guided infraclavicular blocks with 30 mL of 0.25% bupivacaine + 50 mcg fentanyl for group 1 and 30 mL of 0.25% bupivacaine + 100 mcg fentanyl for group 2 were performed before patients emerged from general anesthesia. After surgery, pain levels at rest and during movement were evaluated using the 10-cm visual analog scale (VAS) at recovery room admission, at the 15th and 30th minutes in the recovery room, and at the 2nd, 6th, 12th and 24th hours postoperatively. Both morphine and rescue analgesic requirements were recorded. Sensorial and motor block durations, patient satisfaction, and complications related to the infraclavicular block were recorded. Results: In both groups, no significant difference in VAS pain scores, total morphine and total rescue analgesic requirements, duration of sensorial and motor block, or patient satisfaction were observed. None of the patients experienced any complications. Conclusion: The mixtures of 0.25% bupivacaine + 50 mcg fentanyl and 0.25% bupivacaine + 100 mcg fentanyl showed similar postoperative effects.