BACKGROUND: Lumbar plexus block was used not only for the proposal of lower limb surgery, but also for that of pain control in the pain clinic. The aim of our study was to evaluate the motor, sensory function and the anatomic parameters after posterior approach of the lumbar plexus block with the help of a nerve stimulator. METHODS: 30 patients of low back pain radiating to lower extremities were randomized into two groups. We performed the psoas compartment block guided by C-arm fluoroscope. With the pateints in prone position, a 100 mm insulated needle was introduced at the surface of L5 transverse process. Stimulation of femoral nerve was used to optimize the needle placement at the center of lumbar plexus (Dancing Patella). 1% mepivacaine hydrochloride (0.2 ml/kg) solution in group 1 (n=11) and 0.5% mepivacaine hydrochloride (0.2 ml/kg) solution in group 2 (n=19) were then injected. We measured the minimal current, distance of puncture site from midline, distance from skin to transverse process, distance from skin to the point of maximal twitch, and distance from transverse process to the point of maximal twich. Motor and sensory functions were measured at 5, 15, 30, 45, 60, 90 and 120 min after procedure. RESULTS: Anatomical parameters were as follows; distance of puncture site from midline was 42.6 5.6 mm, distance from skin to transverse process was 52.9 +/- 7.0 mm, distance from skin to the point of maximal twitch was 68.8+/-5.6 mm, and distance from transverse process to the point of maximal twich is 15.9+/-3.9 mm. Difference in attenuated motor function(hip adduction, knee extension) was observed between two groups (P<0.05). Hip adduction and knee extension were recovered 60 min after injection in group 1. In group 2, knee extension was attenuated above 2 hours. We observed significant hip flexion attenuation 90 and 120 min (group 1 and group 2, etc) after injection Sensory blockade was recovered 120 min after injection in both groups. CONCLUSIONS: It is suggested that above mentioned anatomical parameters would be useful for the safe blockade. We found that recovery of attenuated hip flexion is crucial for the discharge criteria of ambulating outpatient with the low back pain.