Chronic obstructive pulmonary disease (COPD) with hypercapnia is associated with increased mortality. Non-invasive ventilation (NIV) can lower hypercapnia and ventilator loads but is hampered by a low adherence rate leaving a majority of patients insufficiently treated. Recently, nasal high flow (NHF) has been introduced in the acute setting in adults, too. It is an open nasal cannula system for delivering warm and humidified air or oxygen at high flow rates (2-50 L/min) assisting ventilation. It was shown that this treatment can improve hypercapnia. The mechanism of reducing arterial carbon dioxide (CO2) is proposed through a reduction in nasal dead space ventilation, but there are no studies in which dead space volume was measured in spontaneously breathing subjects. In our case report we measured in a tracheostomized COPD patient CO2 and pressure via sealed ports in the tracheostomy cap and monitored transcutaneous CO2 and tidal volumes. NHF (30 L/min mixed with 3 L/min oxygen) was administered repeatedly at 15-minutes intervals. Inspired CO2 decreased instantly with onset of NHF, followed by a reduction in transcutaneous/arterial CO2. Minute ventilation on nasal high flow was also reduced by 700 ml, indicating that nasal high flow led to a reduction of dead space ventilation thereby improving alveolar ventilation. In conclusion, NHF assist ventilation through clearance of anatomical dead space, which improves alveolar ventilation. Since the reduction in hypercapnia was similar to that reported with effective NIV treatment NHF may become an alternative to NIV in hypercapnic respiratory failure.