Among the 413 patients of the lumbar herniated nucleus pulposus(HNP) who were admitted and treated at Dong-A University Hospital from March 1990 to May 1994, we analyzed 387 patients who checked magnetic resonance imaging(MRI). MRI study followed by open disectomy was carried out on 127 patients of the lumbar HNP, and we analyzed the clinical and operative findings of the lumbar HNP and compared these with MRI findings. The following results were obtained. 1. MRI findings of 387 patients consisted of bulging discs in 107 cases(27.6%), protruded discs in 165 cases(42.6%), extruded subligamentous discs in 73 cases(18.9%), extruded transligamentous discs in 34 cases(8.8%) and sequestered discs in 8 cases(2.1%). 2. MRI findings of 127 patients who had been treated with open discectomy consisted of protruded discs in 53 cases(41.7%), extruded subligamentous discs in 47 cases(37.0%), extruded transligamentous discs in 19 cases(15.0%) and sequestered discs in 8 cases(6.3%). 3. Operative findings revealed the protruded discs: 49 cases(38.6%), extruded subligamentous discs: 52 cases(40.9%), extruded transligamentous discs: 18 cases(14.2%) and sequestered discs: 8 cases(6.3%). 4. In sequestered disc, we could get the highest sensitivity, specificity and accuracy with MRI in distinguishing from another type of lumbar HNP. MRI can be available for accurate assessment of type and location of the lumbar HNP and it can show the high specificity, sensitivity and accuracy in preoperative diagnostic method.