A rare case of acute septic arthritis of the temporomandibular joint (TMJ) is reported here. A 87-year-old male visited our hospital because of severe pain in the right TMJ region, trismus, eating disorder, and malocclusion. His maximum mouth opening (MMO) was 20mm, and his mandible deviated to the left side 4 mm. Radiographic findings revealed deviation of the right mandible to the anterior-inferior direction. Under clinical diagnosis of acute septic arthritis of the right TMJ, intravenous administration of antibiotics (CEZ: 2 g/day) was started. An MRI was taken in the hospital on day 10, which revealed severe joint effusion in the superior joint compartment. Although we could not obtain pus by puncture from the superior joint compartment, we performed arthrocentesis of the right TMJ. After arthrocentesis, his MMO increased to 40mm, and the malocclusion was cured. We could not detect the origin of the infection in the present case.
