OBJECTIVE: The aim of this prospective longitudinal investigation was to compare the relationship between the temporomandibular joint (TMJ) condyles and the temporal fossae by means of tomography before and after the orthodontic correction of Class II, division 1 malocclusion using the activator appliance. MATERIALS AND METHODS: The final sample consisted of 26 consecutively treated Class II, division 1 patients (19 boys and 7 girls with an average pre-treatment age of 11 years) who underwent orthodontic treatment by means of an activator appliance. Before treatment all patients were free of signs and symptoms associated to TMJ disorders. Bilateral tomographic records before and after treatment were taken and analyzed. Outlines of the condyle and temporal fossa were automatically determined by an edge-detection protocol, and the minimum joint space dimension was automatically measured every 2 degrees. For further analysis, the joint space was divided into anterior, superior, and posterior joint spaces. RESULTS: The average treatment time with the activator appliance was 366 days. In all subjects, activator treatment resulted in a Class I dental arch relationship. After activator treatment, no differences were found in the joint space measurements in any direction. Comparisons between the right and left condyles were not significantly different. CONCLUSIONS: Joint space dimension in Class II division I children was similar before and after treatment in both TMJs.