STUDY DESIGN: A structured literature review. SUMMARY OF THE BACKGROUND DATA: Widely recognized classification criteria for rheumatologic disorders have resulted in well-defined patient populations for clinical investigation. OBJECTIVE: We sought to determine whether similar criteria were needed for back pain disorders by examining variability in eligibility criteria in published studies. METHODS: Studies involving radiculopathy due to lumbar herniated disc (HD) and for neurogenic claudication due to lumbar spinal stenosis (LSS) were identified. Randomized controlled trials published between January 1, 2006 and October 1, 2008 in select peer reviewed journals were retrieved, their eligibility criteria were identified and categorized. RESULTS: Twelve eligible HD studies were identified. Thirteen unique categories of eligibility criteria were identified with a mean of 3.9 (+/-2.0) and a range from 0 to 8 categories per study. More categories were present for studies that included nonsurgical (5.6 +/- 2.5) treatment for studies with only surgical treatment (2.6 +/- 1.7) P = 0.04). Seven LSS studies met eligibility criteria, and 9 unique categories were identified. A mean of 5.0 (+/-2.2) categories with a range from 2 to 7 was used per study. CONCLUSION: Wide variation in the number and type of eligibility criteria from randomized clinical trials of well defined back pain syndromes was identified. These results support the need for developing and disseminating international classification criteria for these clinical conditions.