Background: Lumbar foraminal spinal stenosis (LFSS) is a common cause of radicular pain in the lower extremities. Transforaminal epidural steroid injection (TFESI) is being used widely for controlling radicular pain induced by LFSS. The efficacy of TFESI has been demonstrated in previous studies. However, no study has evaluated the outcome of TFESI according to the severity of LFSS. Objective: In this study, we evaluated the outcome of TFESI in patients with chronic lumbar radicular pain due to LFSS according to the severity of LFSS by using magnetic resonance imaging (MRI). Study Design: A prospective observational study. Setting: A university hospital. Methods: Sixty patients with chronic lumbar radicular pain due to LFSS were included in this prospective study and received TFESI at our university hospital. Three patients were lost to followup. On the basis of sagittal lumbar MRI findings, we assigned patients with mild to moderate LFSS to group A (n = 31) and those with severe LFSS to group B (n = 26). Pain intensity was evaluated using a numeric rating scale (NRS) before treatment and at 1, 2, and 3 months after treatment. Results: Compared to pretreatment NRS scores, a significant decrease in NRS scores was observed in patients in both groups at 1, 2, and 3 months after treatment (P = 0.000). However, reductions in the NRS scores over time were significantly larger in group A (P = 0.023). Three months after treatment, 27 patients (87.1%) in group A and 11 patients (42.3%) in group B reported successful pain relief (pain relief of ≥ 50%). Limitations: This study had a small number of patients. Conclusions: After TFESI, chronic lumbar radicular pain was significantly reduced regardless of the severity of LFSS, and the effects of TFESI were sustained for at least 3 months after treatment. However, the outcome of TFESI was superior in the group with a mild to moderate degree of LFSS, compared to the group with a severe degree of LFSS. We believe that our study provides useful information for establishing a treatment plan for radicular pain due to LFSS. Key words: Lumbar foraminal spinal stenosis, transforaminal epidural steroid injection, disease severity, magnetic resonance imaging, numeric rating scale, corticosteroids