Key words: plasmocytoma / multiple myeloma / oral pathology Abstract -Mandibular plasmocytoma leading to the discovery of a multiple myeloma. Report of a case. Plasmacytoma is considered as the initial presentation of multiple myeloma (MM), a devastating and fatal disease. MM is more frequent in men (male-female ratio = 4:1) and generally occurs between the 5th and the 8th decade, affecting with greater frequency vertebrae, skull, pelvic bones, ribs, humerus and femur. According to the WHO, in 2002, MM was the 17th cause of death by neoplasms in the world. We report a case of a 30-year-old woman, referred to the Maxillofacial surgery pregraduate school of the Faculty of Odontology of national autonomous University (Mexico) for a swelling of her left jaw, with an initial diagnosis of infection, and later histopathologically diagnosed as a plasmacytoma. A complete work-up including blood tests, urine tests and total-body x-rays led to a definitive diagnosis of MM, with a rare histologic profile, considered to be having a poor prognosis. The patient was subjected to chemotherapy but she passed away five months later. This case represents one of those undiagnosed cases of malignant neoplasms, considered first as an unsuspected infective lesion and treated as such with antibiotic therapy. The case also highlights the role that could be played by the dental profession in the early diagnosis of bone cancers, which often have a poor prognosis.Le MM est une néoplasie maligne développée à partir des lymphocytes B, caractérisée par une prolifération de plasmocytes