Background: Transforaminal epidural steroid injection is one of the effective treatments in managing radicular pain. There have been some prospective studies on the depth to the epidural space with the transforaminal approach. However, there have been no studies about the depth in Asians, especially Koreans. This study was carried out in order to evaluate the depth to the epidural space and the oblique angle and factors that influence the depth to the epidural space during lumbar transforaminal epidural injection. Methods: A total of 248 patients undergoing fluoroscopically guided transforaminal epidural steroid injections were evaluated. At the L3-4, L4-5, L5-S1, and S1 levels, we measured the oblique angle and depth to the epidural space. Results: Needle depth was positively associated with body mass index (correlation coefficient 0.52, P = 0.004). The median depths (in centimeters) to the epidural space were 6.13 cm, 6.42 cm, and 7.13 cm for 50-60 kg, 60-70 kg, and 70-80 kg groups, respectively, at L5-S1. Age and height were not significantly associated with the needle depth. Conclusions: There is a positive association between the BMI (and weight) and transforaminal epidural depth but not with age, sex, and height.