STUDY DESIGN: Retrospective study. OBJECTIVES: To observe the short term effect of selective nerve root block (sNRB) depending on the contrast pattern and spinal canal size. SUMMARY OF LITERATURE REVIEW: A number of studies have demonstrated that sNRB is quite effective not only for patients with herniated intervertebral discs but also for those with spinal stenosis. MATERIALS AND METHODS: The Visual Analog Scale(VAS) score was collected before and after the procedure from 217 subjects with lumbar spinal stenosis and underwent sNRB. Two types were classified after observing the contrast's spreading pattern, Type I contrast reaching the spinal canal and Type II not reaching the spinal canal. Efficacy of the treatment for each type was also compared. In addition, the spinal canal size was classified into three categories. Treatment efficacy depending on the contrast pattern was also compared in each category. RESULTS: When divided into two types based on the contrast pattern, type I showed a more significant reduction in VAS score according to T-test although both types showed a decrease in VAS score after the procedure. In regards to spinal canal dimension, both types showed decreased VAS scores after the procedure in patients with spinal canal size larger than 172.2mm2; however, there were no changes in VAS score before and after the procedure for those with spinal canal size smaller than 73mm2. CONCLUSIONS: There was a short term effect of selective nerve root block (sNRB) in patients with spinal stenosis regardless of their contrast pattern, type I group showing a stronger correlation. In regards to spinal canal dimension, patients with larger spinal canal sizes not only showed a significant decrease in VAS score after selective nerve root block (sNRB) but also showed differences depending on the contrast pattern. On the contrary, there was no significant difference in VAS score before and after selective nerve root block (sNRB) in patients with small spinal canal sizes, and there was also no difference in the outcome depending on the contrast pattern in patients with small spinal canal sizes. Therefore, when performing selective root nerve block (sNRB), the operator should remember to manipulate the angle and position of the spinal needle when injecting the appropriate drug after confirming that the contrast material reached the spinal canal. The operator should also consider surgical management when performing selective nerve root block (sNRB) in patients with severe central spinal stenosis.