Non-invasive ventilation is applied with increasing frequency in patients with chronic hypercapnic COPD and insufficiency of the ventilatory pump. In the few existing clinical trials on long-term use of NIV, no significant improvement on survival could be proven, mainly due to methodical reasons. The "National Task Force for Non-invasive ventilation and weaning" plans to study patients with severe COPD and hypercapnic ventilatory pump insufficiency in a prospective, randomised, multicentre clinical trial over one year. In the intervention group, NIV will be applied for at least six hours per day in addition to standard COPD-treatment. The target of mechanical ventilation is a reduction of PCO (2) during spontaneous breathing by at least 20 %, or into the normal range. The main outcome parameter is all-cause mortality, secondary outcome parameters are course of the disease, exercise capacity, quality of life and consumption of medical resources. The sample size is estimated on 300 patients (150 control group, 150 intervention group). The whole study will take approximately three years.