The effectiveness of lumbar fusion for chronic low back pain after surgery for disc herniation has not been evaluated in a randomized controlled trial. The aim of the present study was to compare the effectiveness of lumbar fusion with posterior transpedicular screws and cognitive intervention and exercises. Sixty patients aged 25-60 years with low back pain lasting longer than 1 year after previous surgery for disc herniation were randomly allocated to the two treatment groups. Experienced back surgeons performed transpedicular fusion. Cognitive intervention consisted of a lecture intended to give the patient an understanding that ordinary physical activity would not harm the disc and a recommendation to use the back and bend it. This was reinforced by three daily physical exercise sessions for 3 weeks. The primary outcome measure was the Oswestry Disability Index (ODI). Outcome data were analyzed on an intention-to-treat basis. Ninety-seven percent of the patients, including seven of eight patients who had either not attended treatment (n=5) or changed groups (n=2), completed 1-year follow-up. ODI was significantly improved from 47 to 38 after fusion and from 45 to 32 after cognitive intervention and exercises. The mean difference between treatments after adjustment for gender was -7.3 (95% CI -17.3 to 2.7, p=0.15). The success rate was 50% in the fusion group and 48% in the cognitive intervention/exercise group. For patients with chronic low back pain after previous surgery for disc herniation, lumbar fusion failed to show any benefit over cognitive intervention and exercises.