Objective: This study reports pain and disability outcomes up to 4 years for chiropractic and medical patients with low back pain (LBP) and assesses the influence of doctor type and pain duration on clinical outcomes. Design: Prospective, longitudinal, nonrandomized, practice-based, observational study. Setting: Fifty-one chiropractic and 14 general practice community clinics. Subjects: A total of 2870 acute and chronic ambulatory patients with LBP of mechanical origin. Methods: Sixty chiropractic (DC) and 111 general practice (MD) physicians participated. Primary outcomes were pain, using a 100-point visual analogue scale (VAS), and functional disability, using the Revised Oswestry Disability Questionnaire. These were measured at baseline and 8 time points. Regression analysis compared acute and chronic DC and MD patients after correcting for baseline differences in the 4 cohorts. Results: Most improvement was seen by 3 months and sustained for 1 year; exacerbation was seen thereafter. Acute patients demonstrated greater relief at all time points. A clinically important advantage for chiropractic patients was seen in chronic patients in the short-term (>10 VAS points), and both acute and chronic chiropractic patients experienced somewhat greater relief up to 1 year (P<.000). The advantage for DC care was prominent for chronic patients with leg pain below the knee (P<.001). More than 50% of chronic patients had over 50 days of pain in the third year. Conclusion: Study findings were consistent with systematic reviews of the efficacy of spinal manipulation for pain and disability in acute and chronic LBP. Patient choice and interdisciplinary referral should be prime considerations by physicians, policymakers, and third-party payers in identifying health services for patients with LBP.