Introduction: The McKenzie method (mechanical diagnosis and therapy [MDT]) is an exercise treatment to identify appropriate exercises and postures that improve symptoms and then apply them. This study's aim was to investigate whether MDT was effective for sub-acute low back pain and to analyze factors related to its effectiveness.

Methods: This was a retrospective, cohort study involving 210 patients who had low back pain for >4 weeks and <3 months and received outpatient treatment at our hospital. The participants were divided into two groups according to whether they were treated with MDT (MDT [+], n = 79) or not (MDT [−], n = 131). All patients performed stretching and trunk muscle exercises. Clinical results were compared between the two groups after 1 and 3 months of treatment using a visual analog scale (VAS), Oswestry Disability Index (ODI), and Japanese Orthopaedic Association Back Pain Evaluation Questionnaire (JOABPEQ). Factors related to the effectiveness of MDT were also analyzed.

Results: Each clinical measure was higher after 1 and 3 months of treatment than before in both groups. The MDT (+) group had a significantly higher increase in the score for pain-related disability and a higher efficacy rate for pain-related disability and lumbar dysfunction of the JOABPEQ than the MDT (−) group. The group with the effect of MDT had more severe lumbar dysfunction before treatment than the group without.

Conclusions: In patients with sub-acute low back pain, the MDT (+) group showed significantly better clinical outcomes than the MDT (−) group. This suggested that MDT might be a useful exercise treatment for sub-acute low back pain.