Background: Given the high prevalence of clinically relevant premenstrual symptoms and the associated impairment, there is a need for effective treatments. Initial evidence suggests cognitive-behavioural therapy (CBT) as an effective treatment for premenstrual dysphoric disorder (PMDD). The aim of the current randomized clinical trial was to evaluate an Internet-based CBT (iCBT) to reduce the burden of PMDD. Methods: In all, 174 women with PMDD were recruited via newspaper articles, flyers, and social media. They were randomized to a treatment group (TG; n = 86) or waitlist control group (CG; n = 88). Women of the TG received an 8-week therapist-guided iCBT. Data were assessed before and after treatment/waiting, and 6 months after intervention with prospective symptom diaries and questionnaires in the pre-menstrual phase. Treatment effects and moderators were analysed using hierarchical linear modelling. Results: Significant time x group interaction effects on functional impairment and psychological impairment, impact on everyday life, symptom intensity, and symptom disability in favour of the TG indicated the efficacy of the treatment. Follow-up assessments demonstrated treatment effects to be stable until 6 months after treatment. Additionally, significant interactions with moderator variables were found. In the TG, higher levels of active coping and lower levels of support-seeking coping were associated with stronger improvement in interference in everyday life and symptom intensity. In addition, lower levels of perceived stress were associated with stronger improvement in functional impairment. Conclusion: The iCBT was highly effective in reducing the burden of PMDD. It appears to be particularly important to address coping styles and stress management in the treatment. (C) 2019 S. Karger AG, Basel