Objective: The classification of patients with low back pain (LBP) may be important for improving clinical outcomes and research efficiency. The purpose of this study was to examine the inter-tester reliability of 2 trained physical therapists to classify patients with LBP by using the standardized Movement System Impairment (MSI) classification system. The 5 proposed MSI classifications are based on the most consistent patterns of movement and alignment observed throughout the examination that correlate with the patient's symptom behavior. Design: Test-retest to assess reliability. Setting: Academic healthcare center outpatient facility. Participants: Thirty subjects (21 women and 9 men) with chronic, recurrent LBP (mean age 31.1 ± 12.9 years) were examined independently by 2 experienced physical therapists. Methods: Training consisted of self-study of a procedure manual, a supervised practice of examination procedures and classification rules, and discussion. Subjects were examined independently by each therapist using a test-retest design. Each therapist assigned a LBP classification upon completion of the examination. Both therapists were blinded to the other therapist's findings. Main Outcome Measures: Inter-tester reliability of therapists classifying the LBP problems was indexed by the percent agreement and kappa coefficient. Results: Overall percent agreement on the classification assigned was 83% with kappa = 0.75 (95% confidence interval = 0.51-0.99; P < .0001). Conclusion: Inter-tester reliability of classification of patients with LBP when therapists use a standardized clinical examination based on the MSI classification system is substantial.