A group of 20 subjects who underwent successful Bionator treatment was compared with 20 subjects who were treated less successfully with the same appliance. Both groups had similar advancements in their bite registrations, as well as similar treatment times and growth-prediction parameters. Success was judged not on the final occlusion (all patients were treated to a Class I molar relationship) but on the posttreatment position of skeletal pogonion. The successful group experienced 3.5 mm or more of advancement in skeletal pogonion, whereas the less successful group had less than 3 mm of advancement of this point. The two groups were comparable in all features except, as may be expected, total mandibular growth, which was greater in the group with the larger anterior pogonion advancement. The results of this study suggest that persons who have small mandibles (as determined by comparison with published growth standards) may benefit more from functional appliance therapy than patients with normal-sized mandibles. The subjects with delayed growth may experience more mandibular development than those with average growth during treatment under the favorable growth environment created by functional appliance therapy.