The purpose of this study was to assess whether the tenth rib line (an imaginary line that joins the lowest points of the rib cage on the flanks) could be used as a marker of the lumbar vertebral level. Simple X-rays (n = 100) were taken with radiopaque markers attached on the lowest points of the rib cage and the uppermost points of the iliac crests on both flanks. The spinous process or interspinous space that the tenth rib or Tuffier's lines crossed was identified and recorded, respectively, in the neutral and fully flexed positions. With lumbar flexion, the tenth rib line (median (25th to 75th percentiles)) moved upward (L2 (L1−2 − L2) vs. L1–2 (L1–2 – L1–2); p < 0.01), but Tuffier's line moved downward (L4−5 (L4 − L4−5) vs. L4−5 (L4 − L5); p < 0.01). Because the ease of palpating the tenth rib line and its distribution patterns are comparable to those of the Tuffier's line, the tenth rib line may be useful as a new landmark of the lumbar vertebral level as well as a safeguard to prevent spinal puncture from being mistakenly performed at a dangerously high level.