For years, physical deconditioning has been thought to be both a cause and a result of back pain. As a consequence physical reconditioning has been proposed as treatment-goal in patients with chronic low back pain (LBP). However, it is still unclear whether a patient's physical fitness level really decreases after pain-onset. The objectives of the present study were, firstly, to test the assumption that long-term non-specific LBP leads to a decrease of the level of physical activity (disuse), secondly, to evaluate any development of physical deconditioning as a result of disuse in CLBP, and thirdly, to evaluate predictors for disuse in CLBP. A longitudinal cohort study over one year including 124 patients with sub-acute LBP (i.e., 4-7 weeks after pain onset) was performed. Main outcome measures were change in physical activity level (PAL) and physical fitness (measured by changes in body weight, body fat and muscle strength) over one year. Hypothesized predictors for disuse were: pain catastrophizing; fear of movement; depression; physical activity decline; the perceived level of disability and PAL prior to pain. Results showed that only in a subgroup of patients a PAL-decrease had occurred after the onset of pain, whereas no signs of physical deconditioning were found. Negative affect and the patients' perceived physical activity decline in the subacute phase predicted a decreased level of PAL over one year. Based on these results, we conclude that as to the assumption that patients with CLBP suffer from disuse and physical deconditioning empirical evidence is still lacking.