When he/she flexes and extends the lumbar spine, the lumbar motion segments rotate and translate in the sagittal plane. This coupled motion can be described as a simple rotation about a fixed center lying in each segment. The location of the center can be determined by plotting images of this vertebra in relation to the underlying vertebra in the motion segment. This center is known as the instantaneous axis of rotation(IAR). It has been suggested that degenerative disease and instability can be diagnosed on the basis of abnormally located instantaneous axes of rotation. This study descirbes a method that determines the IAR using lateral radiographs and reports the results for 17 normal individuals and 17 individuals with intervertebral disc herniation or degeneration who were studied at the L4-L5 and L5-S1 segments for the movement of full flexion form fully extended position. Acetate tracings and contour matching techniques recorded the relative positions of the vertebal bodies on each film. Mean IAR between control group and group with intervertebral disc herniation or degeneration. However, position of IAR of L5-S1 n the Y-axis shifted upward compared with L4-L5 in the group with disc herniation or degeneration.