Cognitive variables hypothesised to be mediating obsessive—compulsive behaviour include an overestimation of the probability and consequences of danger, a sense of overinflated personal responsibility for harm to oneself or others, a belief in the overimportance of thoughts, a lack of confidence in memory, an intolerance of uncertainty and a need to control thoughts. In the present study, the relationship between these variables and improvement in OCD symptoms was examined. Fourteen obsessive—compulsives with checking concerns completed a 12-session treatment program of exposure and response prevention (ERP). Before and after treatment, subjects completed the Maudsley Obsessional-Compulsive Inventory (MOCI) and a self-rating of severity (SER), and each week they completed the Yale-Brown Obsessive Compulsive Scale (YBOCS) and answered seven questions that were used to rate their beliefs in the six cognitive domains outlined above. Changes in perception of danger, intolerance of uncertainty and need to control thoughts all correlated significantly with improvement in symptoms. Moreover, significant changes in these variables occurred immediately prior to or concurrent with major symptom improvement, supporting the possibility that they are mediators of the disorder. Changes in ratings of responsibility, overimportance of thoughts and confidence in memory were not related to improvements in compulsive checking on any scale and, in the case of responsibility, ratings actually increased (i.e., worsened) in the week prior to major improvement in symptoms. For most subjects, major reductions in responsibility ratings only took place after symptom reduction. The implications of these findings for the role of these variables in mediating obsessive—compulsive checking are discussed.