Background: The addition of intrathecal (IT) magnesium to spinal fentanyl prolongs the duration of spinal analgesia for vaginal delivery. In this prospective, randomized, double-blind, controlled study, we investigated the effect of adding IT magnesium sulphate to bupivacaine–fentanyl spinal anaesthesia. Methods: One hundred and two ASA I or II adult patients undergoing lower extremity surgery were recruited. They were randomly allocated to receive 1.0 ml of preservative-free 0.9% sodium chloride (group S) or 50 mg of magnesium sulphate 5% (1.0 ml) (group M) following 10 mg of bupivacaine 0.5% plus 25 µg of fentanyl intrathecally. We recorded the following: onset and duration of sensory block, the highest level of sensory block, the time to reach the highest dermatomal level of sensory block and to complete motor block recovery and the duration of spinal anaesthesia. Results: Magnesium caused a delay in the onset of both sensory and motor blockade. The highest level of sensory block was significantly lower in group M than in group S at 5, 10 and 15 min (P < 0.001). The median time to reach the highest dermatomal level of sensory block was 17 min in group M and 13 min in group S (P < 0.05). The mean degree of motor block was also lower in group M at 5, 10 and 15 min (P < 0.001). The median duration of spinal anaesthesia was longer in group M (P < 0.001). Conclusion: In patients undergoing lower extremity surgery, the addition of IT magnesium sulphate (50 mg) to spinal anaesthesia induced by bupivacaine and fentanyl significantly delayed the onset of both sensory and motor blockade, but also prolonged the period of anaesthesia without additional side-effects.