Objectives: To compare the long-term effectiveness of surgical and non-surgical treatment in patients with chronic low back pain. Methods: Two merged randomised clinical trials compared instrumented transpedicular fusion with cognitive intervention and exercises in 124 patients with disc degeneration and at least 1 year of symptoms after or without previous surgery for disc herniation. The main outcome measure was the Oswestry disability index. Results: At 4 years 14 (24%) patients randomly assigned to cognitive intervention and exercises had also undergone surgery. 15 (23%) patients assigned fusion had undergone re-surgery. The mean treatment effect for the primary outcome was 1.1; 95% CI -5.9 to 8.2, according to the intention-to-treat analysis and -1.6; 95% CI -8.9 to 5.6 in the as-treated analysis. There was no difference in return to work. Conclusions: Long-term improvement was not better after instrumented transpedicular fusion compared with cognitive intervention and exercises.