Aim: The aim of this study was to investigate the effect of a cognitive behavioural therapy intervention provided by trained cardiac nurses plus usual care compared with usual care alone in patients with an implanted cardioverter defibrillator who, prior to randomization, had presented with symptoms of anxiety measured by the Hospital Anxiety and Depression Scale. Background: Around 20–25% of all patients with an implanted cardioverter defibrillator experience anxiety. Among these patients anxiety is associated with mortality, presumably explained by risk behaviour and activation of the autonomic nervous system. We hypothesised that cognitive behavioural therapy provided by cardiac nurses in an out-patient setting would be an effective treatment of anxiety. Methods: This was an investigator-initiated randomised clinical superiority trial with blinded outcome assessment, with 1:1 randomisation to cognitive behavioural therapy plus usual care or to usual care. The primary outcome was Hospital Anxiety and Depression Scale-Anxiety mean score at 16 weeks. Secondary outcomes included Hospital Anxiety and Depression Scale-Depression, Becks Anxiety Inventory, HeartQoL and Hamilton Anxiety Scale. Primary outcome analysis was based on the intention-to-treat principle. Results: A total of 88 participants were included, 66% men, mean age 64.3 years and 61% had primary indication implantable cardioverter defibrillator. A significant difference after 16 weeks was found between groups: intervention group (Hospital Anxiety and Depression Scale-A mean (standard deviation) 4.95 points (3.30) versus usual care group 8.98 points (4.03) (p < 0.0001)). Cohen's d was –0.86, indicating a strong clinical effect. The effect was supported by secondary outcomes. Conclusion: Cognitive behavioural therapy provided by cardiac nurses to patients with a positive screening for anxiety had a statistically and clinically significant effect on anxiety compared with patients not receiving cognitive behavioural therapy.