A 51-year-old woman was referred to a periodontology practice for treatment of advanced generalized periodontitis stage IV, grade C. When the dental hygienist treated the distal surface of tooth 17 with the ultrasonic scaler, the patient indicated she heard a painful, high tone. The dental hygienist immediately stopped the treatment. At a subsequent attempt to treat the same distal surface, the patient heard a painful, high tone again. After two weeks, the high tone had not disappeared. The patient was referred to an ENT specialist. He confirmed the diagnosis: tinnitus in the right ear. Pharmacological treatment with prednisone did not produce a positive result. Ultrasonic treatment of periodontitis is generally considered safe. However, in exceptional cases, sudden deafness and/or tinnitus can occur. In the event of such complaints, it is recommended not to wait, but to refer the patient to an ENT specialist immediately.