The pathogenesis of reduced mandibular dimensions and asymmetry was studied in 103 patients with juvenile rheumatoid arthritis. Abnormalities of the temporomandibular joint were intimately related to mandibular growth disturbances. Unilateral joint abnormality resulted in jaw asymmetry and underdevelopment on the affected side. Duration of disease and of corticosteroid therapy, age at disease onset, and age when the data were recorded were correlated with the size of the mandible. Our findings did not support a hypothesis of cervical spine involvement as an etiologic factor of reduced mandibular size.