The preoperative MR findings in 11 patients, all of whom had developed recurrent low back pain after surgery for herniated lumbar invertebral disk, were correlated with the surgical findings to determine possible criteria for distinguishing recurrent disk herniation from postoperative scar (extradural fibrosis). The preoperative MR findings agreed with the surgical findings in seven of eight patients with recurrent disk herniation and in six of nine individuals with extradural fibrosis. The most important parameters in differentiating recurrent herniated disk from extradural scar were the configuration and margination of the extradural mass rather than its signal characteristics. The most reliable MR sign for recurrent herniated disk was the presence of a sharply marginated focal polypoid disk protrusion beyond the posterior margins of the adjacent vertebral bodies shown to best advantage on sagittal T1- and T2-weighted and axial T1-weighted spin-echo MR images. Disk herniations usually maintained isointensity with the intervertebral disk of origin, while extradural fibrosis exhibited variable signal intensity. The preoperative diagnosis of extradural fibrosis on MR was based primarily on its irregular configuration and extension. This study suggests that preoperative differentiation between scar and recurrent herniated disk is possible with MR when morphology and topography are considered in addition to signal intensity.