This study was conducted on lateral cephalograms and magnetic resonance images (MRIs) obtained from 20 subjects with Class III malocclusion. Only clinically temporomandibular joint (TMJ) symptom-free subjects were included in this study. In the treatment group, a chin cup with 600 g of force was applied in 13 patients (10 girls and three boys) with a mean age of nine years. The chin cup was applied in a direction from the chin toward the TMJ. The control group consisted of seven patients (six girls and one boy) with a mean age of eight year nine months. Orthodontic treatment was not applied in the control group. Records were taken at the beginning and end of chin cup therapy from all the subjects. Measurements were made on lateral cephalograms and unilateral-left sagittal-oblique TMJ MRIs. Variables obtained at the beginning and end of the study were compared by Student's t-tests and paired t-tests. Relationships between craniofacial and TMJ variables were analyzed by correlation analysis. The mandibular corpus length was increased and condylar head angle was decreased by chin cup therapy. A positive correlation existed between activation of sagittal maxillary and mandibular growth and bending of the condylar head. This study showed that the condylar growth pattern was altered by chin cup. It may be implied that the source of improvement is adaptation of the craniofacial structures to the changes of the condylar growth pattern produced by the chin cup.