Background: Guided internet-delivered CBT for anxiety disorders has received increasing empirical support, but little is known regarding the role of the therapist. Aims: This study addressed therapist factors in guided internet-delivered cognitive behavioural therapy for anxiety disorders. Method: Data from three controlled trials with a total N of 119 were analyzed with attention to differences between eight therapists. Results: No significant mean level differences between therapists appeared in the dataset. However, one significant intraclass correlation between participants was found, suggesting that the outcome on the Beck Anxiety Inventory might have been influenced by the impact of the individual therapists. Conclusion: The therapist can possibly have some influence on the outcome of guided internet-delivered CBT for anxiety disorders, but studies with more statistical power are needed to establish whether therapist effects are present in this modality of psychological treatment. The present study was underpowered to detect minor therapist effects. © British Association for Behavioural and Cognitive Psychotherapies 2010.