BACKGROUND: The relationship between the injection speed of a local anesthetic and the success rate of unilateral spinal anesthesia has been a controversial issue. The aim of this thesis was to identify any significant effects of the drug injection speed on the success rate of unilateral spinal anesthesia. METHODS: Forty patients were randomly allocated into 2 groups, group R and S. The injection speed was 4 ml/min in group R (n = 20) and 1 ml/min in group S (n = 20). Hyperbaric 0.5% bupivacaine 10 mg was injected via a syringe pump. The drug was administered at the L3-4 intervertebral space with the patient in the lateral decubitus position, which was maintained for 20 minutes after the injection. A spinal sensory block was assessed by examining the temperature sensation using an alcohol-sponge. The motor block was evaluated using the modified Bromage scale and the dependent and non-dependent sides were compared. RESULTS: Significant differences (P<0.05) were observed in the success rate of unilateral motor paralysis (45% in group R vs 90% in group S). There were no significant blood pressure differences between the two groups 5, 10, 15, 30 and 60 minutes after injecting the hyperbaric 0.5% bupivacaine. CONCLUSIONS: The injection speed of local anesthetics is one of the crucial factors for achieving a unilateral spinal anesthesia. Therefore, it is important to maintain a slow injection speed of a local anesthetic in unilateral spinal anesthesia.