This study evaluates the condylar response to activator in growth and in maturing patients using radiographs and magnetic resonance images (MRI). Seven patients (four in growth and three in maturing stages) treated for mandibular distal occlusion were studied. In all seven patients, lateral roentgen cephalograms, panoramic radiograms, and MRIs were made before and following functional treatment. All patients' mandibles advanced during treatment. Downward and forward mandibular growth was observed by superimposition of lateral cephalograms. On the condylar posterosuperior regions for both groups, double contours were sometimes observed on the panoramic radiograms following therapy. These double contours coincided with an area of high intensity in the MRIs for both groups. In the mature adult group, the double contours were more clearly observed when compared with those in the growth group. There were differences in the condylar adaptation types between the growth and mature development stages. Condylar adaptation to the newly created mandibular position was nevertheless found even in adult patients.