BACKGROUND: The subcutaneous ICD (S-ICD™) is an important advance in device therapy for prevention of sudden cardiac death (SCD). In some patients, decision pro- or contra-ICD implantation is particularly challenging due to inconsistent data on risk of ventricular tachyarrhythmias or sudden cardiac death, rare entities, special medical or family history, or patients' wishes. Whether decision-making in these borderline cases has been facilitated with the new option of a S-ICD™ is unknown. MATERIAL AND METHODS: All patients with an implanted S-ICD™ without a class I or IIa recommendation for primary prophylaxis of sudden cardiac death in the current guidelines (n = 30 patients) in our large-scaled single-centre S-ICD™ registry (n = 249 patients) were included in this study. Baseline characteristics, appropriate and inappropriate shocks, and complications were documented in a mean follow-up of 40 months. RESULTS: In all patients S-ICD™ implantation was performed for primary prevention of SCD. Of all 30 patients with an overall mean age of 40.5 ± 15.6 years, 17 were male (57%). The mean left ventricular ejection fraction (LVEF) was 54.5 ± 9.9%. Indication were highly variable and ranged from structural heart disease, nsVT and LV-EF > 35% to patients with polymorphic non-sustained ventricular tachycardia (nsVT) and suspect syncope. During follow-up, six episodes of sustained ventricular tachyarrhythmias and four episodes of ventricular fibrillation (VF) were adequately terminated in three patients (10%). Two of these patients were implanted for polymorphic nsVT and previous syncope without structural heart disease. In three patients, T-wave-oversensing and in one patient also P-wave-oversensing resulted in an inappropriate shock (five in total), two additional episodes of oversensing ended before shock delivery. There were no S-ICD™ system-related infections. In five patients S-ICD™ replacement was performed due to battery depletion (four regular, one premature). In five patients, ablation procedures were performed after implantation (four because of frequent symptomatic ventricular extra beats, one because of atrial flutter). Change to a transvenous system was necessary in two patients due to need for antibradycardia pacing. CONCLUSION: The use of the S-ICD™ was safe in patients with borderline or unclear indication for ICD implantation in our study. Of note, during a relatively short mean follow-up there were several appropriate therapies, especially for VF in these patients. On the other hand, oversensing also occurred in about 10% of patients, while lead problems were not problematic in this collective. S-ICD™ implantation may be considered as a possible alternative in cases of borderline indications and clinical uncertainty when decision pro-ICD implantation is made. Incidence of arrhythmias was quite high and mostly consisted of VF. Nevertheless, patient education seems even more important as there is a considerable risk for inappropriate therapies as well.