Ankylosing spondylitis (AS) is a chronic inflammatory disease that affects mainly the axial skeletal system, causing pain and functional incapacity. The peripheral joint involvement occurs in 30 to 40% of cases. Osteomyelitis of the mandible was relatively common before the advent of preventive antibiotic therapy and restorative dentistry. Currently, the infection of the facial bones is a rare condition, being the odontogenic infection the most responsible for cases with mandibular involvement. The authors report the case of a EA patient, with severe peripheral involvement, which progressed to osteomyelitis of the jaw, secondary to the odontogenic infection due to delay in diagnosis and treatment.