Purpose: To evaluate whether patients' treatment preferences, characteristics, or symptomatic response to assessment moderated the effect of the McKenzie method for acute low back pain (LBP). Methods: This study involved a secondary analysis of a previous RCT on the effect of adding the McKenzie method to the recommended first-line care for patients with acute non-specific LBP. 148 patients were randomized to the First-line Care Group (recommended first-line care alone) or the McKenzie Group (McKenzie method in addition to the first-line care) for a 3-week course of treatment. The primary outcome was pain intensity at 3 weeks. The ability of six patient characteristics to identify those who respond best to McKenzie method was assessed using interaction terms in linear regression models. Results: The six investigated potential effect modifiers for response to the McKenzie method did not predict a more favorable response to this treatment. None of the point estimates for effect modification met our pre-specified criterion of clinical importance of a 1 point greater improvement in pain. For five of the six predictors, the 95% CI did not include our criterion for meaningful clinical improvement. Conclusion: We were unable to find any clinically useful effect modifiers for patients with acute LBP receiving the McKenzie method.