PURPOSE: To evaluate the relationship between the direction of herniated disc and pressure effect on nerve root and dural sac, as seen on MRI. MATERIALS AND METHODS: We retrospectively reviewed lumbar spine MR images of122 cases of lumbar disk herniation in 75 patients MRI findings were analyzed with regard to the relationship between the direction of the herniated disc and pressure effect on nerve root and dural sac. pressure effect onnerve root and dural sac was arbitralily divided into three types. Type I was defined as zero or minimal compression of nerve roots or thecal sac by the herniated disc ; type II was defined as mild to moderate compression, while III was defined as severe compression or displacement of nerve roots and/or thecal sac. RESULTS: Of the 122 cases seen in these 75 patients, 97(80%) were observed at L4-5 and L5-S1. The central type(71cases;58%) was more frequently observed than the posterolateral type(48cases; 40%) or lateral type(3cases; 2%). Thetotals of types I, II, and III were 44(36%), 43(35%), and 35 cases(29%), repectively. Seventy-seven %(34/44) oftype I and 65%(28/43) of type II were of the central type but for type III, the corresponding figure was only 26%(9/35). CONCLUSIONS: On MR imaging, most of lumbar disk herniations were observed at L4-5 and L5-S1, with apredominance of the central type rather than the posterolateral one, Most of the central types were either type Ior type II.