Background: Recently, there have been several case reports and retrospective studies about the incidence of intradiscal (ID) injection during transforaminal epidural steroid injection (TFESI). Inadvertent ID injection is not a rare complication, and it carries the risk of developing diskitis, although there has been no report of diskitis after TFESI. We prospectively evaluated the incidence of inadvertent ID injection during lumbar TFESI and analyzed the contributing factors. Methods: Ten patients received 2-level TFESI, and the remaining 229 patients received 1-level TFESI. When successful TFESI was performed, 2 ml of contrast dye was injected under real-time fluoroscopy to check for any inadvertent ID spread. A musculoskeletal radiologist analyzed all magnetic resonance images (MRIs) of patients who demonstrated inadvertent ID injection. When reviewing MRIs, the intervertebral foramen level where ID injection occurred was carefully examined, and any anatomical structure which narrowing the foramen was identified. Results: Among the 249 TFESI, we identified 6 ID injections; thus, there was an incidence of 2.4%. Four patients had isthmic spondylolisthesis, and the level of spondylolisthesis coincided with the level of ID injection. We further examined the right or left foramen of the spondylolisthesis level and identified the upward migrated disc material that was narrowing the foramen. Conclusions: Inadvertent ID injection during TFESI is not infrequent, and pain physicians must pay close attention to the type and location of disc herniation.