OBJECTIVES: Obesity is often considered as a risk-factor for higher complication rates and worse clinical outcome of fusion surgery in the lumbar spine and is therefore sometimes not performed in obese patients despite relative indication for surgery. The goal of this study was to evaluate whether BMI is a predictive factor for clinical outcome after monosegmental fusion surgery in the lumbar spine. METHODS: The present study is a retrospective analysis of prospectively collected data in a consecutive series of patients. Between April 2002 and April 2007, a total of 467 patients underwent monosegmental lumbar fusion in a single spine center. Preoperatively, at 6 weeks and at 1 year follow-up, SF36 and Oswestry-Disability-Index scores were collected. We excluded patients who underwent surgery due to infections, tumor and trauma, as well as revision surgeries, and all patients with incomplete datasets, so that 223 patients were included in the study. Of those patients, variables considered as risk-factors like age, BMI and the presence of comorbidities like diabetes mellitus were assessed from the medical records. A multiple regression model for those parameters and clinical outcome was cretated. RESULTS: In an unadjusted model, BMI did not at all predict clinical outcome, in a multivariate model adjusted for baseline outcome values of SF36, Oswestry-Disability index and age, a slight trend towards negative correlation between BMI and outcome could be shown (p=0.06) at one-year follow-up. CONCLUSIONS: This study suggests that BMI alone is not a good predictor of clinical outcome of monosegmental lumbar fusion one year after surgery and that therefore this kind of surgery should not be withheld from patients only because of obesity.