The severity of lesions in the temporomandibular joint (TMJ) area and their association with age at onset, the various forms of juvenile rheumatoid arthritis (JRA), and certain serologic tests for rheumatoid factor (RF), antinuclear antibodies (ANA), and human leukocyte antigen (HLA)-B27 were determined in 15-yr-old children. The series comprised 121 JRA children, 78 girls and 43 boys, in whom an analysis had recently been made of the relation of TMJ lesions to jaw movement and occlusal status. The mean age at onset was 7.3 yr, the girls contracting JRA earlier than the boys. Also, the girls with lesions in the TMJ area were significantly younger than those with no lesions. TMJ abnormalities were found in 50% of cases with a pauciarticular or systemic onset, but in 72% of those representing the polyarticular subtype. Flattened condyles and grave lesions were equally represented in all subgroups and in both sexes. A crossover from onset type to present diagnosis was found in 30% of the cases, mostly from pauciarticular to polyarthritis, which also increased the risk of TMJ lesions from 50 to 60%. RF, ANA, or HLA-B27 alone did not seem to be associated with a risk of TMJ abnormalities. Maximal opening capacity is more restricted in patients with early onset or a polyarticular mode of JRA. Since the TMJ is affected in more than half of JRA children, regular measurements of maximal movements of the mandible or roentgenologic examinations of the TMJ are essential for their optimal treatment.