Aim: Unilateral spinal anaesthesia permits early recovery and short ambulatory stay. Our study aimed to search if meperidine may prolong sensory block time when added to hyperbaric bupivacaine. Methods: This is a prospective, double blinded study: Ambulatory, 46 consenting patients aged 18-60 years, undergoing unilateral knee arthroscopy were randomized in two groups. saline group (n=20): 1.3 ml of hyperbaric bupivacaine and 0.2 ml of serum physiologic was used. Meperidine group (n=20): 1.3 ml of hyperbaric bupivacaine and 0.2 ml of 5% meperidine was used. Sensory block times, duration of spinal anaesthesia, intraoperative adverse effects and patient satisfaction were recorded. Results: Mean duration of sensory block was greater in the meperidine group compared with the saline group. Strict unilateral block and hypotension were comparable among groups. Conclusion: Addition of meperidine to hyperbaric bupivacaine in unilateral spinal anaesthesia prolonged analgesia without effecting total anesthesia time with minimal adverse effects.