Aplasia of the mandibular condyle alone without any other facial malformation and medical history is a very rare condition. This clinical report describes treatment with a functional appliance in a young patient with aplasia of the mandibular condyle alone without any other facial malformation, which is a very rare condition. The patient, a 6(1/2)-year-old girl, was referred for treatment of mandibular asymmetry. There was no relevant family history. The patient showed early mixed dentition with a II molar relationship on the right and II on the left side, 5 mm of overbite and 5 mm of overjet, and a lower midline discrepancy of 3 mm. According to the Pruzansky-Kaban classification of mandibular deformity, the patient was a type IIA; according to Vento and his classification, the mandible of the patient was M2B. The patient was given a Haupl-Andresen activator, which had been modified to reposition the right mandible downward and forward. The functional appliance therapy lasted for approximately 4 years. The affected side showed remarkable condylar growth compared with the normal side. Seven years later, the correction of the mandibular asymmetry was stable and no relapse had occurred.