Abstract: Most alveoli could collapse in different illnesses and injuries to a lung. A relatively high positive pressure, which could in itself damage the lung or impede its healing, is required to open the alveoli. A positive pressure is often applied at the end of the expiratory phase in order to keep the alveoli open throughout entire respiratory cycles. A ventilator is disclosed wherein an opening pressure for the lung can be determined from measurement values for the pressure and the volume of air supplied to the lung. The ventilator registers the measurement values and determines the opening pressure from the relationship between measurement values for pressure and for volume. The determined opening pressure can then be used by a physician for setting an appropriate end expiratory pressure (PEEP) for the respiratory phase or be used by a control unit in the ventilator for automatic setting of an appropriate end expiratory pressure.