OBJECTIVE: Investigate the influence of external factors such as depression and BMI among subjects with primary severe low back pain (LBP) and low back related leg pain (LBLP). BACKGROUND: The report of disability in patients with LBP may be significantly influenced by confounding and moderating variables. No similar studies have examined the influence of these factors on LBLP. METHODS: This study included 1,448 consecutive subjects referred to a tertiary spine clinic. Unconditional binary logistic regression was used to determine the influence of comorbidities on the relationship between self-reported back and leg pain. A change in estimate formula was used to quantify this relationship. RESULTS: Among those subjects with primary LBP the unadjusted odds ratio was 8.58 (95% CI 4.87, 15.10) and when adjusting for BMI, depression and smoking was 5.94 (95% CI 3.04, 11.60) resulting in a 36.7% change due to confounding by these comorbidities. Among those with primary LBLP, the unadjusted odds ratio was 4.49 (95% CI 2.78, 7.27) and when adjusting for BMI and depression was 4.60 (95% CI 2.58, 8.19) resulting in a 1.7% change due to confounding by these comorbidities. CONCLUSION: The disability statuses of the patients with primary LBP in this study were more significantly affected by comorbidities of BMI, depression and smoking than patients with report of LBLP. However, these comorbidities contribute little to the relationship of primary low back related leg pain and Oswestry scores ≥ 40.