To examine whether patients with chronic low back pain exhibit changes in cognitive factors following Interactive Behavioural Modification Therapy (IBMT), delivered by physiotherapists; and to examine the association between pre- to post-treatment changes in cognitive factors (cognitive processes) and pre- to post-treatment changes in pain, disability and depression.Observational before–after study.Outpatient physiotherapy department.One hundred and thirty-seven patients with chronic low back pain.IBMT: ‘Work Back to Life’ rehabilitation programme.Pre- to post-treatment changes in pain, disability and a range of cognitive factors.Patients demonstrated significant favourable changes for a range of cognitive factors. Furthermore, pre- to post-treatment changes in these cognitive factors explained an additional 22%, 17% and 15% of the variance in changes in pain, disability and depression, respectively, after controlling for other important factors.Changes that emerge in cognitive factors are strongly related to treatment outcome within a physiotherapy treatment context. Specifically, reductions in fear of movement and catastrophising, and increases in functional self-efficacy appear to be particularly important. Modifying these cognitive factors should be seen as a priority when treating patients with chronic low back pain.