Radiographic findings of the upper and lower jaw bone of 20 adult patients with micrognathia, bird face, and juvenile rheumatoid arthritis are reported. In all patients a symmetrically underdeveloped mandible with the chin posteriorly positioned was found at cephalometry. Arthritic lesion of the temporomandibular joint, mostly symmetric, with limitation of movement and secondary arthrosis, was observed in all patients. Complete absence of the tubercle. Abnormal anterior position of the mandibular head occurred in almost half of the patients. The degree of mandibular growth disturbance seemed to be correlated to the severity of the arthritis, indicating the arthritis to be a causal mechanism of micrognathia.