BACKGROUND The dosage of local anaesthetic and the time the patient must be kept in the lateral decubitus position for a unilateral spinal anaesthesia is not known. The aim of this study was to determine the ideal dosage of hyperbaric bupivacaine and the time required for the lateral decubitus position for a unilateral spinal block. METHODS Ninety patients who were scheduled to receive spinal block for surgery in the lower extremity were randomised into 9 groups (n = 10). The spinal block was performed through the L4-L5 intervertebral space with the patient in the lateral decubitus position. Patients in groups Ia, Ib, Ic; IIa, IIb, IIc; IIIa, IIIb, IIIc received 1.5 ml of 0.5%, 2 ml of 0.5%, and 2.5 ml of 0.5% hyperbaric bupivacaine solutions, respectively. The patients were turned to the supine position for 5 min after the injection in groups Ia, IIa, IIIa, 10 min after the injection in groups Ib, IIb, IIIb, and 15 min after the injection in groups Ic, IIc, IIIc. The onset and regression of sensory and motor block were checked and compared between the dependent and non-dependent sides in each group. RESULTS The rate of block progression of the non-dependent side was higher in the groups receiving 2.5 ml 0.5% hyperbaric bupivacaine solution than in the other groups; at the same time the level of block was higher and the duration of block was longer. The incidence of hypotension was 10-20% in these groups. In the 2 ml 0.5% hyperbaric bupivacaine solution groups, a satisfactory block level and duration of anaesthesia for surgery was obtained. The rate of block progression to non-dependent side in the groups receiving 1.5 ml of 0.5% hyperbaric bupivacaine solution was lower than the other groups, but the duration of block was shorter and the level of block was lower than the other groups. CONCLUSIONS For unilateral spinal anaesthesia in lower extremity operations, 2ml 0.5% hyperbaric bupivacaine solution for operations above the knee and 1.5 ml 0.5% hyperbaric bupivacaine solution for operations below the knee and keeping the patients for 10 min in the lateral decubitus position were found to be appropriate.