Abstract: A process is disclosed for thermally obliterating biological tissues by laser radiation introduced into the tissue by means of an optical waveguide. The laser radiation is scattered by means associated with the radiation output surface of the optical waveguide. A biocompatible, medium to highly visquous liquid which does not substantially absorb the laser radiation but scatters it, is injected into the tissue, forming a scattering fluid deposit around the radiation output surface which is not separated from the tissue and which allows the tissue to be heated in a controlled manner.