OBJECTIVE--To compare chiropractic and hospital outpatient treatment for managing low back pain of mechanical origin. DESIGN--Randomised controlled trial. Allocation to chiropractic or hospital management by minimisation to establish groups for analysis of results according to initial referral clinic, length of current episode, history, and severity of back pain. Patients were followed up for up two years. SETTING--Chiropractic and hospital outpatient clinics in 11 centres. PATIENTS--741 Patients aged 18-65 who had no contraindications to manipulation and who had not been treated within the past month. INTERVENTIONS--Treatment at the discretion of the chiropractors, who used chiropractic manipulation in most patients, or of the hospital staff, who most commonly used Maitland mobilisation or manipulation, or both. MAIN OUTCOME MEASURES--Changes in the score on the Oswestry pain disability questionnaire and in the results of tests of straight leg raising and lumbar flexion. RESULTS--Chiropractic treatment was more effective than hospital outpatient management, mainly for patients with chronic or severe back pain. A benefit of about 7% points on the Oswestry scale was seen at two years. The benefit of chiropractic treatment became more evident throughout the follow up period. Secondary outcome measures also showed that chiropractic was more beneficial. CONCLUSIONS--For patients with low back pain in whom manipulation is not contraindicated chiropractic almost certainly confers worthwhile, long term benefit in comparison with hospital outpatient management. The benefit is seen mainly in those with chronic or severe pain. Introducing chiropractic into NHS practice should be considered.