An apparatus for the thermosurgical treatment of biological tissue comprises a generator (22) for providing treatment energy, measuring means (24) for registering the temporal progression of a measured quantity that is influenced by the tissue impedance of the treated body or representative of the tissue impedance, and also a control unit (26) which has been set up to ascertain a frequency spectrum within a predetermined examination frequency range for the temporal progression of the measured quantity and to control the energy output of the generator (22) in a manner depending on the spectral content of the measured quantity within the examination frequency range. The invention is based on the finding that the genesis of vapor bubbles in the heated tissue can be detected on the basis of the frequency spectrum of the tissue impedance, in particular within a frequency range between approximately 0.5 kHz and 200 Hz. Depending on the extent of the vapor bubbling, the tissue impedance displays a differing spectral image within this frequency range. In accordance with the invention, this finding is utilized for the purpose of controlling the energy output of the generator.
