BACKGROUND: When attempting to identify lower lumbar spaces by palpating the spinous processes and iliac crests, the actual level often turns out to be higher than presumed. The rib cage line that joins the lowest points of the rib cage on the flank can be physically constructed in the same way as Tuffier's line. The purpose of this study was to assess the possibility that the rib cage line could be used as a marker for the lumbar spine level. METHODS: After IRB approval, 100 patients aged 20 to 69 were enrolled. The patients were stratified into 10 groups according to age and gender. Simple X-rays previously taken in the lateral neutral and lateral flexed positions were reviewed. In the lateral films, the rib cage line was drawn to connect the two lowest points of the tenth rib and Tuffier's line was drawn to the two uppermost points of the iliac crest such that the lines were perpendicular to the tangential line on the skin at the intersection points. Then, the vertebral body or intervertebral space, at which each line crossed, was identified and recorded respectively in the neutral and flexed positions. RESULTS: With maximum flexion of the lumbar spines, the vertebral level (25% to 75% range), at which the rib cage line crosses, appeared to be between the upper third of L1 and the upper third of L3. For Tuffier's line, the vertebral level was between the L3-4 intervertebral space and the lower third of L5. When the lumbar position was changed from neutral to flexed position, the rib cage line shifted cephalad (P < 0.05) and Tuffier's line shifted caudad (P < 0.05). CONCLUSIONS: We conclude that the rib cage line may be used as another marker of vertebral level and that it can act as a safeguard in that the spinal block should be performed one level lower than the rib cage line to prevent the spinal puncture from being performed at an undesirably high level.