A device for treatment of body tissue by means of a light diffuser (13) circumferentially and endoluminally irradiating said tissue by laser light energy, wherein said diffuser is connected at its proximal end to a source of laser light energy via a flexible wave guide comprising a fiber optic core (1) covered by an optical cladding (2) and a protective sheath (14). The distal end of the wave guide has its protective sheath at least partially removed and is provided with grooves adapted to refract and/or reflect the light propagating within the core in generally radial directions. A cap (7) transparent to the laser light encloses the distal end of the core. The grooves comprise at least two spiral grooves (4, 5) extending through said optical cladding into said core, successive grooves of the respective spiral grooves are alternating along the longitudinally extending outer surface of the core and its optical cladding.
