Objective. To determine whether psychological characteristics predict outcome and/or response to physiotherapistdirected exercise- or advice-based treatment of subacute low back pain. Methods. We conducted a secondary analysis of a factorial, placebo-controlled trial (n = 259). The psychological characteristics were catastrophizing, coping, pain self-efficacy, fear of injury/movement, depression, anxiety, and stress. We used mixed models to predict pain and function outcomes (both scored on a 0-10 scale). The models include a term for treatment group, a term for the psychological characteristic (which tested prediction of outcome), and an interaction term between the treatment group and psychological characteristic (which tested treatment effect modification). To aid the interpretation of the magnitude of the effect modification, we calculated the change in outcome for a 1 SD increase of the baseline score of the putative effect modifier. A ≥1.5-point change of the outcome of interest per 1 SD change of putative effect modifier was regarded as clinically important. Results. All of the psychological characteristics except coping predicted outcome, but none appeared to be important treatment effect modifiers. Only 5 of the 56 tests of treatment modification were statistically significant, and none of the 95% confidence intervals (95% CIs) for the interactions included clinically important effects. For example, a 1 SD higher baseline level of anxiety was associated with a 0.62 (95% CI 0.10, 1.15) additional effect of exercise on function at 52 weeks. Conclusion. Most of the psychological characteristics we tested predicted outcome, but none predicted response to physiotherapist-guided exercise and/or advice.