Objective
The purposes of this study were to evaluate the relation of the lumbar plexus with the psoas compartment, to measure the distance from skin to psoas compartment, and to determine the efficacy of psoas compartment block for the unilateral leg pain and/or low back pain.

Method
Six cadavers were dissected and the computed tomography of the lumbar region were performed in 22 subjects. The psoas compartment block of 10 ml of 0.5 percent lidocaine were performed in 31 patients with unilateral leg pain and/or low back pain. Visual Analog Scale (VAS) and sensory, motor functions were assessed before the block and 5, 30 minutes, and 1, 2, 3 hours and 1 week after the block. The distance from skin to psoas compartment were measured during the procedure. The questionnaires on the procedure were completed after 1 week.

Results
Cadaver dissections demonstrated that the 3 main nerves of lumbar plexus were within the psoas compartment between the level of L4 and L5. Computed tomography provided that the average distances of anterior and posterior borders of psoas major from the low back skin were 10.8 & 6.3 cm and that of medial & lateral borders from the median sagital plane were 2.9 & 7.1 cm respectively. There were statistically significant correlations between distance from skin to psoas compartment and body weight, abdominal circumference as well as body mass index (p＜0.0001). The VAS was 7.7 before the block and 5.4 1 week after the block (p＜0.05) and satisfactory outcomes were shown in 71 percent of the subjects.

Conclusion
Psoas compartment block was effective for the patients with unilateral leg pain and/or low back pain without major side effects and complications.