The histologic and histochemical changes in back muscle were studied in virgin surgery patients with lumbar spine disorders and in patients who underwent repeat posterior lumbar surgery. The results were correlated to provide the evidences of histologic changes of back muscle after posterior lumbar surgery. Back muscles were examined histologically and histochemically after posterior lumbar surgery. No previous study has assessed these changes. Back muscles were obtained before and after retraction from 18 virgin surgery cases with lumbar spine disorders. In four patients, the retraction pressure was monitored and the retraction pressure-time products ([P][T]) were calculated. In 21 repeat lumbar surgery cases, muscle samples were obtained before muscle retraction. Samples were evaluated by histologic and histochemical methods. Abnormal findings were slight in virgin surgery cases. Early back muscle injury tended to depend on operation time and ([P][T]) products. Late back muscle injury in reoperated patients was marked. Various types of neurogenic changes were observed more than 10 months after the first operation. Histologic damages of back muscle due to previous surgical intervention were long-lasting. To avoid permanent muscle injury, the retraction time and pressure should be shortened or the pressure on the back muscle should be monitored during posterior surgery.