The formation of a new joint between a pathologically enlarged coronoid process and the body of the malar bone is known as Jacob's disease. hyperplasia of the coronoid process was first described in 1853 by von Langenbeck, and it was not until 1899 when Oscar Jacob described the disease that was named after him. Jacob's disease is an uncommon entity with only a few cases documented in the literature. the condition manifests at first with progressive limitation of the oral opening and facial asymmetry. the pain is infrequent and mainly affects young patients. temporal muscle hyperactivity, cranial trauma, chronic displacement of the ipsilateral temporomandibular joint, endocrine stimuli and genetic alterations have been postulated as possible factors. the definitive diagnosis is by histopathology and it is necessary that bone hyperplasia is confirmed, as well as the presence of cartilage and synovial capsule forming the new joint between the malar bone and the coronoid process. we present a 10-year-old patient with a history of childhood trauma in the left preauricular region. it presented to our service with a history of progressive limitation of the oral opening. computed tomography (CT) revealed an elongation of the bilateral coronoid process, in contact with homolateral zygomatic bone, causing its deformation. surgery under general anesthesia was performed through the intraoral vestibular route. histopathology confirmed the diagnosis of Jacob's disease. we review the literature regarding the etiology, pathogenesis, clinical characteristics, diagnosis and treatment of this condition.