Background: The impact of ICD therapy on patient well being has typically focused on mean differences between groups, thereby neglecting changes within individuals. Using an intra-individual approach, we examined (i) the prevalence of implantable cardioverter defibrillator (ICD) patients maintaining their pre implantation level of psychological functioning at 12 months, and (ii) factors associated with deterioration in functioning. Methods: Consecutively implanted ICD patients (n=332) completed a set of standardized and validated patient reported measures at baseline and at 12 months post implantation. Results: The majority of patients (72.8% to 81.7%) preserved their pre implantation level of psychological functioning 12 months post implantation. In adjusted analysis, ICD shock (all ps<.001) and Type D personality (all ps<.05) were independent predictors of deterioration in psychological functioning at 12 months across all domains, while baseline psychological status was associated with an improvement (all ps<.05). Patients with a primary prevention indication experienced a decrease in ICD co Conclusions: The majority of ICD patients maintained their pre implantation level of psychological functioning at 12 months. A subset of patients was at risk of poor psychological adaptation, attributable to ICD shocks, Type D personality, atrial fibrillation, and left ventricular dysfunction, while primary prevention indication and older age had a protective effect against deterioration in functioning. (C) 2011 Elsevier Ireland Ltd. All rights reserved.