The invention relates to a medical instrument for treating tissue by means of high-frequency current. The inventive instrument comprises at least one electrode that can be impinged upon with a high-frequency current and that is disposed on the distal end of an elongated electrode carrier. At least one electromagnetically active position sensor is disposed in the distal zone of the electrode carrier in the direct vicinity of the at least one electrode in such a manner that it detects the intracorporeal position and/or location of the at least one electrode. The invention further relates to a medical system that comprises such an instrument.