BACKGROUND: Lumbar plexus block by posterior approach has been used for thigh and hip surgery and unilateral low back pain and/or low extremity pain control. We measured the needle depth from the skin to the lumbar plexus and its relation with body indices in Korean. METHODS: Forty-eight (male 35, female 13) patients with unilateral low back pain and/or low extremity pain received lumbar plexus block by the posterior approach known as "psoas compartment block". We measured the distance from the skin to the lumbar plexus with the aid of a nerve stimulator and evaluated its relation to body mass indices such as weight, height, abdominal circumference and body mass index (BMI; kg/m2). RESULTS: The distance described above was 7.42 +/- 0.82 cm (range 6.0-9.5 cm) in males and 7.18 +/- 1.24 cm (range 5.3-10.0 cm) in females. Its relation to body weight was greater than with other body indices. The Pearson's coefficient between the distance and body weight was 0.745 in males and 0.842 in females. The predicted distance (cm) was 4.23 + 0.0471 x weight (kg) in males (adjusted R2 = 0.532) and 1.25 + 0.0975 x weight in females (adjusted R2 = 0.682). CONCLUSIONS: Body weight is the most important factor for prediction of the distance from the skin to the lumbar plexus. The distance was about 6-10 cm in korean patients.