Background: In clinical practice, we have often observed contrast material spreading into the epidural space during lumbar facet joint injection. However, the exact incidence of epidural spread and contrast pattern have not been reported. Purpose: To evaluate the incidence and pattern of epidural spread in lumbar facet joint injection. Material and Methods: One hundred consecutive patients (38 men, 62 women; mean age 68 years; age range 20–88 years) who underwent lumbar facet joint injection at two sites between April 2014 and June 2014 were investigated in this prospective study. Initial oblique, final anteroposterior, and lateral fluoroscopic images were obtained and evaluated for the presence of epidural spread and its contrast pattern (based on direction and extent) with the consensus of three radiologists. The relationship between epidural spread and its potential predictors was analyzed using the chi-squared test, Fisher’s exact test, and the t-test. Results: The incidence of epidural spread during lumbar facet joint injection was 64.6% (n=64) in 99 patients and 49.5% (n=95) in 192 procedures; ventral spread occurred in 29.2% and foraminal spread in 18.8%. When epidural spread occurred, the most commonly identified distributions were unilateral (73.7%), dorsal (92.6%), and cephalad (92.6%). Epidural spread increased significantly in the caudocephalic direction (P < 0.0001), in men (relative risk [RR]=1.478), in the adjacency of posterior fusion level (RR=1.545), in patients with spondylolisthesis (RR=1.454), and when there was no other leakage (RR=0.334). Conclusion: Epidural spread occurred at about half the number of lumbar facet joint injections and showed a contrast pattern similar to that seen with the interlaminar approach.