PURPOSE: To determine morphologic change in a herniated lumbar disc, as seen on MR imaging, according to position change (supine and flexed-prone positions). MATERIALS AND METHODS: A hundred and twenty-two patients with herniated lumbar discs , as seen on supine MR imaging, underwent repeat MR imaging in the flexed-prone position; 100 patients, who showed more than 2 ° of flexion angle difference, were included in this study. Sixty-two were men and thirty-eight were women, and their ages ranged from 13 to 59 (mean, 30) years. Disc degeneration was graded as 1, 2, or 3, depending on the area of decreased signal intensity seen on a T2-weighted MR image. Difference in the angle of flexion is defined as the angle difference of lumbar curvature, calculated by the Begg-Falconer method in supine and flexed-prone positions. Morphologic changes in herniated discs in different positions were analysed on the basis of shape change of anterior epidural spaces, thecal sacs and posterior margins of herniated discs, and classified as either A (not changed) or B (changed). Group B was subtyped as type I (decreased herniation without change of shape), type II (decreased herniation with change of shape) or type III(increased disc herniation). We statistically analysed correlation between a patient's age, morphologic change in a herniated disc in different positions, and degree of degeneration in such a disc, as well as the correlation between morphologic change in a herniated disc in different positions and the degree of the flexion angle. RESULTS: Disc degeneration was apparent in 99 of 100 patients. Grades 1, 2, and 3 were seen in 32 (32.3%), 49 (49.5%), and 18 (18.2%) patients, respectively; the difference in the angle of flexion ranged from 2 to 24 ° (mean 7.97; SD=6.20). Group A consisted of 37 (37.4%) patients, and group B of 62 (62.6%) (type I in 29 (29.3%), type II in 27 (27.3%), and type III in 6 (6.0%)). There was no significant statistical correlation between a patient's age and morphologic change in a herniated disc in different positions (correlation coefficient=0.0183; p=0.086), and between the degree of degeneration of a herniated disc and morphologic change in such a disc in different positions (correlation coefficient=0.1736; p=0.249). The mean angle of flexion was 5.15 ° (SD=4.94) in group A and 10.06 ° (SD=6.28) in group B; there was significant statistical correlation between this and morphologic change in a herniated disc (p=0.0017). CONCLUSION: In the flexion-prone position, which is similar to that used during surgery, MRI can reveal various morphologic changes in a herniated lumbar disc, mainly decreased herniation, and is helpful in making decisions related to surgery.