A literature review  on the epidemiology and economic burden of non-specific chronic low back pain reported estimates of prevalence ranging from 6% to 11%. In a cross-sectional survey with 9267 respondents, average total back pain costs per patient per year in Germany have been reported as €1322 . Chronic low back pain (CLBP) seems to account for the majority of these expenses, with annual direct costs of > €7000 per patient .
Imaging studies have revealed that pain is accompanied by an extensive reorganisation of the brain. Changes in chronic back pain patients are structural [3, 4] and functional  and reversible if the pain subsides [6–8].
If a chronic pain conditions is regarded as non-specific  and does not provide a peripheral tissue target for treatment, the medical treatment approach needs to be directed towards altering these central mechanisms by e.g. prescribing opioids and antidepressants. Surgical interventions include implantation of electrical stimulators in the brain. Deep brain stimulation [10–12], and motor cortex stimulation [13, 14] have demonstrated pain reduction.
Transcranial direct current stimulation (tDCS) is a non-invasive alternative that applies weak electrical currents (1-2 mA) through the skull to modulate the activity of neurons in the brain .
A current systematic review of the literature (Luedtke et al., Clin. J. Pain, accepted) concluded that anodal tDCS had a pain reducing effect in patients with chronic pain due to spinal cord injury [16, 17], fibromyalgia [18, 19], chronic pelvic pain , multiple sclerosis  and various chronic pain conditions [22, 23] when applied with an intensity of 1-2 mA over the motor cortex for 20 minutes on a minimum of 2 consecutive days. However, the level of evidence was rated as "low". A risk of bias assessment of the 8 published trials showed that only one trial was of an overall low risk of bias . Four trials met the minimum criteria for the inclusion in the meta-analysis of the results [16, 18, 21, 23]. Although effects for pain were reported as statistically significant across all trials, the pooled effect of -2.29 with a 95% confidence interval of -3.5 to -1.08 only just reached minimal clinically important difference recommendations.
A trial with high methodological quality is needed to determine whether tDCS is effective in the reduction of pain in chronic pain patients.
This trial will evaluate the effect of tDCS on pain and disability of patients with non-specific CLBP and investigate whether tDCS as a prior treatment enhances the symptom reduction achieved by a cognitive-behavioural group intervention.