A 73 year-old man consulted for a swelling of the endentulous horizontal branch of the mandible. X-ray examination showed a lytic defect. Two successive, but superficial biopsies, directed the diagnosis towards osteomyelitis. In spite of antibiotic therapy, the destruction continued to progress and was finally complicated by a pathological fracture. A radial resection of the horizontal branch of the mandible was performed; on this surgical specimen the diagnosis of primitive lymphoma of the mandible (diffuse centroblastic type) was made. The authors discuss the diagnostic difficulties of mandibular lymphomas, which according to Ostrowski represent 4.7 per cent of the primitive bone lymphoma.