A pilot randomised controlled trial of guided self-help cognitive behaviour therapy for perfectionism: Impact on diagnostic status and comorbidity

•Guided self-help cognitive-behaviour therapy for perfectionism demonstrated efficacy.•Large effects were found for perfectionism and dichotomous thinking.•Medium effects were found for depression and quality of life.•Depressive symptoms were reduced but primary diagnosis of depression remained.•In the treatment group pre to post frequency of comorbidity reduced 69%–13%.