OBJECTIVE: To analyze the influence of body mass index (BMI) on subjective and objective measures of pain, functional impairment, and health-related quality of life in patients with lumbar degenerative disc disease undergoing surgery. METHODS: Prospective institutional review board-approved 2-center study, measuring visual analog scale (VAS) back and leg pain, Roland-Morris Disability Index (RMDI), Oswestry Disability Index (ODI), EuroQol 5D questionnaire, and Short Form-12 at baseline, 6 weeks, 6 months, and 1 year postoperatively. T-scores of objective functional impairment (OFI) were determined using the Timed Up and Go (TUG) test. RESULTS: A total of 375 patients with a median BMI of 26.6 kg/m2 (94 obese patients [BMI ≥30 kg/m2]) were included. Obese patients presented more VAS back pain (mean, 4.7 vs. 3.6; P = 0.001) and greater disability on the RMDI (mean, 12.6 vs. 11.3; P = 0.045). The prevalence and severity of OFI were similar in obese and nonobese patients. There was a weak positive correlation between BMI and VAS back pain (r = 0.1552; P = 0.0026), on both RMDI (r = 0.1138; P = 0.0276) and ODI (r = 0.1075; P = 0.0374). There was no correlation between BMI and TUG T-scores (r = 0.0475; P = 0.3585). Obese patients were as likely as nonobese patients to show a positive 6-week treatment response, and the outcome up to 1 year was similar. CONCLUSIONS: BMI positively correlates with VAS back pain, RMDI, and ODI. Standardized TUG T scores reflect the patient's degree of OFI well, irrespective of BMI. The TUG test appears to be a good means to estimate functional impairment in populations with a high prevalence of obesity.