BACKGROUND: Isobaric bupivacaine has same baricity as cerebrospinal fluid and therefore, so it remains at the level of injection. But, the risk of high spinal anesthesia exist, because increased intrathecal pressure is possible in prone position as isobaric bupivacaine has mild hypobaricity at body temperature but is isobaric at room temperature. So, we studied the influence of the position of the blockade of spinal anesthesia in isobaric spinal anesthesia. METHODS: We studied 26 patients undergoing elective surgery for which spinal anesthesia was considered appropriate. One group (the P group) were scheduled for surgery in the prone position with a frame (n = 13), the second group (the S group) were scheduled for surgery in the supine position (n = 13). Patients were injected with 12 mg of 0.5% isobaric bupivacaine at L3-4 in the lateral decubitus position with a 22 G spinal needle at the rate of 0.2 ml/sec. We then assessed anesthetic blockade level, heart rate, and blood pressure. RESULTS: The height of the sensory block in the prone position group was at the 10th thoracic dermatome, whereas in the supine position this was at the 8th thoracic dermatome at 15 minutes. There was a little difference between the two groups, but this was insignificant statistically. CONCLUSIONS: Both the supine and the prone positions are suitable for isobaric spinal anesthesia with bupivacaine. Isobaric spinal anesthesia in the prone position with a frame is as safe as in the supine position. Spinal anesthesia with isobaric bupivacaine is considered to have a low risk of high spinal anesthesia and a low complication level in the prone position with a frame, as for the supine position.