A method of automatically controlling a respiration system for proportional assist ventilation with a control device and with a ventilator. An electrical signal is recorded by electromyography with electrodes on the chest in order to obtain a signal uemg(t) representing the breathing activity. The respiratory muscle pressure pmus(t) is determined by calculating it in the control unit from measured values for the airway pressure and the volume flow Flow(t) as well as the patient's lung mechanical parameters. The breathing activity signal uemg(t) is transformed by means of a preset transformation rule into a pressure signal pemg(uemg)(t)) such that the mean deviation of the resulting transformed pressure signal pemg(t) from the respiratory muscle pressure pmus(t) is minimized. The respiratory effort pressure ppat(t) is determined as a weighted mean according to ppat(t)=a·pmus(t)+(1−a)·pemg(t), where a is a parameter selected under the boundary condition 0≦a≦1.