Periostitis ossificans(“Garré's osteomyelitis”) of the mandible is a rather rare pathology that occurs as a hard swelling at the mandibular angle, persists for a long period, and is mostly painful on palpation. Clinical signs of infection are rarely noted. The etiological factor is generally a carious first permanent molar with a periapical infection or a fracture of the jaw. In this case, a 10 year‐old girl showed a fixed painful swelling of the right mandibular angle. This revealed to be periostitis ossificans, although none of the classical causes were present. The symptoms were evoked by a periodontal infection: there was an osseous crater and a probing pocket depth of more than 10 mm disto‐lingually of the first right permanent molar of the mandible. This was likely the etiological factor. Extraction of the first and second permanent right molar, curettage of the corresponding alveoli, and antibiotic treatment resulted in complete healing, clinically and radiologically, after 9 months.J Periodontol 1993; 64:60‐62.