Study Design and Objectives The effects of nerve root retraction in posterior discectomy on pressure, blood flow, and electrophysiologic changes of the nerve root were investigated. Objectives This study was performed to assess the effects of nerve root retraction in posterior discectomy on pressure, blood flow, and electrophysiologic changes of the nerve root. Summary of Background Data The effects of compression on nerve tissue have been clinically studied mainly in peripheral nerves. The potential risk for lumbar nerve root injury has not been evaluated precisely in humans. Methods Among 31 patients with lumbar intervertebral disc herniation, the correlation between retraction pressure and changes in blood flow of the nerve root was evaluated. Retraction pressure was continuously monitored during the discectomy and the length of the discectomy procedure was measured. Threshold and amplitude of compound muscle action potentials were monitored before and after discectomy. The relationship between intraoperative data on nerve root physiology and postoperative neurologic change was analyzed. Results Nerve root blood flow was decreased to 18–30% of the initial value under the pressure around 70 g/cm2. An abrupt decrease in nerve root blood flow was observed even within 1 or 2 mm of retraction distance. The threshold of the nerve root before retraction was minimal in the nerve root of the asymptomatic side, but increased in the affected root demonstrating sensory loss only, and significantly increased in the root showing sensory/motor deficit. Amplitude of compound muscle action potentials evoked by the stimulation at distal from herniated mass was lowest in the nerve root showing sensory/motor deficit. In 4 of 31 patients, high retraction pressure (>170 g/cm2) was required and they showed temporary sensory deterioration after surgery. The length of discectomy procedure in these 4 patients was 14.1 ± 4.1 minutes, but it was 9.9 ± 2.7 minutes in the remaining 27 patients. Conclusions In posterior lumbar discectomy, nerve root ischemia usually occurs during retraction. The magnitude of pressure and time of retraction may be potential risk factors for nerve root injury.