To compare the motion of lumbar spondylolisthesis between the standing and decubitus positions and to identify the radiological parameters concerning segmental instability, the lumbar sagittal motion of 5 volunteers and 14 patients was recorded using video-fluoroscopy in both positions. Still images at three frames per second were captured, and the motion pattern was compared between the two positions. Patients were classified into two subgroups according to the degree of postural slip reduction. In the volunteer group, the motion patterns were not particularly different between the two positions. In the small postural reduction group, the motion patterns were not particularly different between the two positions, except in one patient for whom the range of motion (ROM) at the upper adjacent level increased in the decubitus position. In the large postural reduction group, a motion pattern change was observed. The ROM at the slip level in three patients increased, and in the remaining four patients the angle at the midpoint in ROM at the slip level shifted to the extension side in the decubitus position. This condition (anterior slip being reduced in the supine position, with disc angle and average disc height increased) is thought to result from anterior column deficiency and the vertical laxity of the surrounding soft tissues. A biomechanically stable fusion technique is recommended for surgical management.