Introduction: Perfectionism may be a clinically relevant problem on its own or as a part of other conditions. Internet-based cognitive behavioral therapy (ICBT) is a promising method for treating perfectionism. ICBT with guidance is recommended over unguided versions; still there remain questions concerning the importance of guidance in ICBT.

Methods: In this study, seventy-eight self-referred participants were randomized to either ICBT with support or ICBT with support on request in an eight-week treatment of perfectionism. Primary outcome measures included two subscales from Frost Multidimensional Perfectionism Scale, and the Clinical Perfectionism Questionnaire. In addition to this depression, anxiety and quality of life were assessed.

Results: A mixed effects model revealed significant pre-post reductions on all measures for both groups. Neither significant differences nor considerable effects were detected between groups (Cohen’s d = 0.01-0.33).

Conclusion: This trial demonstrates that ICBT with or without regular therapist support may be helpful in treating perfectionism.