Eighty-nine normal subjects were examined with lateral functional radiographs of standing, flexion and extension position using Putto' method. A specially designed radiopaque ruler was put on each film for calibration. Computer-assisted method was used for digitization and analysis. The mean values (standard deviation) of flexion-extension rotation in L1-L2, L2-L3, L3-L4, L4-L5 and L5-S1 were 6.9 (3.1), 13.9 (4.1), 12.6 (4.4), 14.4 (4.0), and 11.8 (6.0) degrees, respectively. The mean values (standard deviations) of flexion-extension translation in L1-2, L2-3, L3-4, L4-5 and L5-S1 were 1.3 (0.09), 1.5 (1.4), 2.2 (1.2), 2.0 (1.3) and 0.4 (0.17) respectively. From flexion to extension, the segmental rotation approached to zero degree and the intersegmental translation changed from retrolisthetic to neutral position although the changes were small. For the flexion-extension rotation, although the large variations may limit some clinical usefulness in the judgement of instability, the data still provided good diagnostic basis if the frequency was taken into consideration. For example, for translation changes, 2 mm was regarded as acceptable in most cases at the levels from L1 to L5 and in the level of L5-S1, the average translation change was only 0.4 mm.