In spite of numerous clinical and patho-anatomical studies made in the past, there are still different opinions concerning the machanism of low back pain. We have focused attention on the posterior structures as an alternative source of low back pain with leg radiation, so we have studied and analyzed the effect of the injection of mixture methylprednisolone acetate suspension (20-40 mg) and local anesthetic (1% procaine, 1cc) into the posterior facet joint. In this paper, the records of 41 in-patients who have chiefly compalned of low back pain wlth leg radiation and have been treated at the Departmnt of Orthopedics, Han Yang University Hospital from May, 1979 to April, 1980 were studied. The following results are recognized by observing and analyzing their chief complalnts, physical examination, x-ray findings, the effect of facet block and the follow-up studies after facet block. 1. Low back pain with sciatica was 1.9 times more common in female than male, and frequently occured In 4th to 6th decades (75.7%). 2. The abnormal findings of plain x-rays were osteophyte (35.6%), marginal sclerosis (27.1%), narrowing of disc spce (18.6%), lumbarization (11.9%), spina bifida (3.4%), and tropism (3.4%). 3. The most frequent site of abnormal posterior facet joints was at the level between 4th and 5th lumbar vertebra (42.9%), and all were on lower lumber region. 4. The range of lumbar motion was decreased in the cases of abnormal posterior facet jolnt or degenerative changes, and it was especially more decreased in the combined cases. 5. The effect of facet block was as follows; In initial assessment, 29 of 41 cases (70.7%) showed complete rellef and one month later, 16 of 29 cases (55.2%) showed continuous rolief, 4 of 18 cases (22.2%) which were followed for 3 more months showed complete relief. 6. The effect of facet block according to pain character was as follows: 20 of 26 cases (76.9%) of numbness, 5 of 6 cases (83.3%) of dull pain and 4 of 8 cases (50.0%) of radlating pain were completely relleved and a case of burning pain was partially relleved.