Lung volume measurement can be of interest to anesthesists as well as respiratory physicians, both as a diagnostic tool and as a way to monitor the effect of respiratory therapy (for instance PEEP ventilation). Several methods exist to measure lung volume. Only two of them can be applied easily during mechanical ventilation. The wash-in wash-out of insoluble inert gas can be used in non-rebreathing (semi open) systems, or the equilibration of an inert insoluble gas which can be used in completely closed systems. In our department a completely closed anesthesia system has been developed which is particularly well suited for the equilibration method of lung volume measurement. Just like the wash-out method, the equilibration method can give information about the uniformity of ventilation-distribution within the lung.