STUDY DESIGN: This is a retrospective study for radiographically and clinically assessing nonunion after lumbar spine fusion. OBJECTIVES: We wanted to analyze the risk factors for nonunion that requires surgical treatment after lumbar spine fusion SUMMARY OF THE LITERATURE REVIEW: A diagnosis of the nonunion after lumbar spine fusion was made by using only the only radiologic images. The incidence of nonunion has been underreported because there are many asymptomatic patients. MATERIALS AND METHODS: The plain X-ray films were evaluated for 1317 patients who could be followed up more than 1 year after lumbar fusion. Nonunion was diagnosed at 1 year after fusion by instability seen on the flexion-extension radiograph and the clinical findings like as sustained pain and local tenderness at the surgical site. The risk factors we reviewed included age, the number of levels fused, associated diseases, smoking, alcohol drinking, the initial diagnosis, a previous history of spinal operation, infection, a clear zone and malposition of pedicle screws and metal failure. The relations between nonunion and the factors mentioned above were analyzed. RESULTS: Thirty-nine patients were diagnosed as having nonunion underwent reoperation and all had surgically confirmed nonunion. Smoking, infection and a previous history of spine operation had a significant influence on nonunion (p < 0.05). Clear zones persisting more than 1 year and metal failure also had a significant influence on nonunion (p < 0.05). Age, the number of fused levels, the initial diagnosis and alcohol drinking were not shown to influence the rate of nonunion (p < 0.05). CONCLUSION: A through clinical and radiologic evaluation is essential to diagnose nonunion, and this should not be done according to the radiologic images only. Risk factors such as a previous history of spine operation, infection, smoking, the development of a clear zone and metal failure all showed a statistically significant influence on nonunion. Additionally, preoperative and postoperative evaluation of these parameters is needed to achieve bone union.