OBJECTIVES: To investigate whether exercise alone or as a part of a multidisciplinary treatment reduces sick leave in patients with non-specific non-acute low back pain. METHODS: A meta-analysis of randomized controlled trials was performed. A qualitative analysis of the sick leave results was performed applying pre-defined levels of evidence. In studies comparing exercise with usual care, pooled effect sizes were computed. RESULTS: Fourteen trials were identified allowing 22 comparisons between treatments. The qualitative and the quantitative analysis showed strong evidence that exercise reduces sick days during the first follow-up year, the effect size (95% confidence interval) was -0.24 ( -0.36, -0.11). In a subgroup of studies on the treatment of severely disabled patients (> 90 sick days under usual care) the effect size was -0.30 (-0.42, -0.17). The effect size of the number of patients receiving a disability allowance was small and not significant. CONCLUSION: The reviewed trials provide strong evidence that exercise significantly reduces sick days during the first follow-up year.