Mandibular advancement devices (MADs) are used in the treatment of obstructive sleep apnea. A patient (woman in the 60-ies) was treated with a MAD (SomnoDent Fusion Flex; SomnoMed AG). She experienced reactions in form a burning sensation in the mouth, ulcers on the tongue and gingiva, and swelling of the tongue. Allergy test (patch test) with Dental Screening Series showed a strong positive reaction to methyl methacrylate, and clear reactions to ethylene glycol dimethacrylate (EGDMA) and 2-hydroxyethyl methacrylate (HEMA).