Objective To determine how often the line joining both iliac crests intersects the spine at L3,4 interspace or L4 spinous process.Methods One hundred and eighty-four patients aged 24-82 yr weighing 50-80 kg scheduled for spine surgery were placed in left lateral position with their hips and knees flexed.A line joining both iliac crests was drown.A metal marker was placed at the point where the intercristal line intersects the spine and detected by mobile X-ray system.The intersection was considered accurately located when the marker lay on L3,4 interspace or L4 spinous process.The rate of inaccuracy was calculated.Results The intercristal line lay between L2,3 interspace and L5 spinous process and lay more cephalad in male than in female.The inaccuracy rate was 18％.Conclusion The location of intersection of intercristal line and spine should be checked cautiously before lumbar puncture.