Exercise therapy has proven to be an effective additive therapy in patients with cardiovascular diseases. The prognostic value of physical activity is well established in patients with congestive heart failure. Therefore the investigators assumed that the population of patients with implantable cardioverter-defibrillators (ICD) with impaired left ventricular function may also benefit from a cardiovascular training in terms of improved quality of life and reduction of ventricular arrhythmia. The data on feasibility, risk and therapeutic effects of exercise training are very limited or not available.