Allergic and irritant contact reactions to face masks for anaesthesia have rarely been reported. We present a 55-year-old female patient who developed facial allergic contact dermatitis after an operation requiring general anaesthesia. Patch tests showed positive reactions to cocospropylenediamin-guanidinium-diacetate (trivial name Dodigen 3558), a preservative used in disinfectants for medical instruments. It could be proven that residues of the causative allergen in the disinfectant adhered to the mask. This is the first report of a clinically relevant sensitization to this increasingly widely used agent.