Background and objectivesPerfectionism is associated with the development and maintenance of anxiety, mood, and eating disorders. Cognitive behaviour therapy (CBT) results in significant reductions in perfectionism and psychological symptoms however, the majority of studies have not examined clinical samples or impact on diagnostic status and comorbidity. MethodsForty participants with elevated perfectionism, of whom 80% had a range of psychological disorders, were randomised to CBT for perfectionism or waitlist control. ResultsThe treatment group reported significant large effect size reductions in perfectionism measures of personal standards and concern over mistakes (d = 1.1–1.6), dichotomous thinking (d = 1.2), and medium effect size reductions in depression (d = 0.55) and increases in quality of life (d = 0.73), which were maintained at 4 month follow-up. The participants in the treatment group were significantly less likely to present with comorbid psychological disorders at post-treatment than the control group. LimitationsThe sample size was small, and there was no control condition at follow-up. ConclusionsFindings lend further support for CBT for perfectionism as an efficacious transdiagnostic treatment for perfectionism and related psychopathology.