The present study investigated whether MI is a mechanism for change in the treatment of obsessive–compulsive disorder (OCD). The Magical Ideation scale (MI), the Obsessive–Compulsive Inventory — Short Version (OCI-SV) and the Padua Inventory were completed by 34 obsessive–compulsive patients pre- and post cognitive–behavioural treatment. Treatment did not target magical styles of thinking. Significant improvements on all three measures of obsessive–compulsive symptoms were demonstrated by t tests over the course of treatment. Improvement in magical thinking was also shown to be significant in t test results. In support of the hypothesis, correlations between MI improvement and improvement on the obsessive–compulsive symptom scales were significant (at a level of .05) suggesting that there is an association between improvement in magical thinking and improvement in obsessive–compulsive symptoms. Notably, a significant negative correlation was obtained between prescores on MI and change scores on the OCD measures. This suggests that high levels of MI are associated with high levels of treatment intractability. High MI appears to be a poor prognostic factor in OCD.