Selective lumbosacral radiculography and nerve root block techniques are very useful in determining the nerve root involved. We have done the lumbar root block to 61-year-old female who had suffered from low back pain radiating to right lower leg which was not relieved by epidural steroid injection two times. $L_5$ root block was performed under the fluoroscopic C-arm guide. When the needle was in correct position, we injected contrast medium (Isovist$^{(R)}$ - 300, Schering, Germany). After we injected 1.5 cc isovist, the S1 root was figured but L5 root was not figured. When we reinjected 1.5 cc isovist, $S_1$ root was enhanced and $L_5$ root was slightly visible due to severe disc bulging and lateral spinal stenosis.