Behavior therapy [exposure and response prevention (ERP)] and cognitive therapy (CT) have proven effective in the treatment of obsessive-compulsive disorder. Direct comparisons between these treatment modalities have exposed no differences in efficacy. However, very little research has been conducted into the differences between the change processes in ERP and CT. This investigation is a first attempt to study change by measuring scores on a weekly basis rather than at specific stages in the treatment and follow-up.