BACKGREOUND: Tuffier's line has been a guide for lumbar puncture. Usually lumbar puncture or epidural anesthesia was performed in the lateral decubitus position with the "forehead-to-knees" position. The purpose of this study was to identify the accuracy with which the spinal level could be predicted from this external mark in the "forehead-to-knees" position. METHODS: Two hundred and twenty-four patients (112 male and 112 female patients) undergoing investigation for back pain were examined. The standard antero-posterior lumbar spine film was taken in the supine position. The lateral lumbar spine film was taken in the lateral decubitus with the "forehead-to-knees" position. These films were examined after being reported upon by a radiologist. The iliac crest was identified and a horizontal line drawn between the highest points using a ruler. The level of Tuffier's line of each age group in supine or "forehead-to-knees" position and relationship with aging were observed. RESULTS: The point coincided with the L4-5 interspace (61%), L4 (20%), and L5 (19%) in the supine, L4-5 (48%), L5 (46%), L4 (5%), and L5S1 (0.4%) in the "forehead-to-knees" position. The Tuffier's line of men were higher than women. The Tuffier's line in the "forehead-to-knees" position went higher with aging in women. CONCLUSIONS: The Tuffier's line was most frequently the L4-5 interspace in supine and "forehead-to-knees" positions. The Tuffier's line in the "forehead-to-knees" was lower than the supine position. The Tuffier's line of men were higher than women of all age groups in the supine position. The Tuffier's line in the "forehead-to-knees" position went higher with aging in women. The Tuffier's line in the supine position in women and of both positions in men did not show any relation with age.