Penetration into the abdominal cavity with injury to visceral, vascular, or neural structures is a potentially life-threatening complication of lumbar disc exploration. In this report, we used reconstructed computed tomographic measurements of the L3-L4, L4-L5, and L5-S1 intervertebral discs to show that lateral spine roentgenograms contain an unavoidable magnification error that makes accurate measurement of minimal depth to penetration impossible. This magnification error may mislead the unwary surgeon into an overestimation of actual disc size. Moreover, the "safe distances" that have been published for the avoidance of penetration at the L3-L4, L4-L5, and L5-S1 intervertebral levels would have been excessive for 22.2%, 19.4%, and 8.3%, respectively, of the 26 patients in our study.