It is believed that patients' expectancies about the effectiveness of treatment influence their treatment outcomes, but the working mechanism is rarely studied in patients with low back pain. Theoretical models suggest that adherence to treatment may be an important pathway. The aim of this study was to assess the mediating role of adherence to treatment in the relationship between expectancies and the outcomes of recovery and pain intensity in patients with acute low back pain. This study used data from a randomized placebo-controlled trial of paracetamol for acute low back pain. Expectancies were measured with the Credibility Expectancy Questionnaire. Adherence was measured with a medication diary. Pain intensity was recorded daily in a diary on a 0 to 10 pain scale, and recovery was defined as the first of 7 consecutive days scoring 0 or 1 on a 6-point pain scale. Cox regression (dependent variable: recovery) and linear mixed-model analyses (dependent variable: daily pain intensity scores) were performed. The "difference in coefficients" approach was used to establish mediation. A total of 1573 participants were included in current analyses. There was a small but highly significant relationship between expectancies and outcomes; 3.3% of the relationship between expectancies and recovery and 14.2% of the relationship between expectancies and pain intensity were mediated by adherence to treatment. This study does not convincingly support the theory that adherence is a key pathway in the relationship between treatment outcome expectancies and recovery and pain intensity in this acute low back pain population.