Abstract: The invention relates to a mouthpiece and method for antibacterial treatment of intraoral surfaces. The mouthpiece comprises a body made of light-guiding material, the body comprising facial and lingual outer surfaces and facial and lingual inner surfaces adapted to face facial and lingual surfaces of teeth, respectively. A light source is attached to the body and adapted to deliver light to surfaces of teeth, the light source being positioned on at least one of said outer surfaces and adapted to deliver said light via said inner surfaces to both the facial and lingual surfaces of the teeth, wherein a portion of light is adapted to travel through said body from the facial side to the lingual side, or vice versa.