The purpose of this study was to compare the effects of a differently designed functional appliance (the R-appliance) with a twin-block (TB)-treated group. Thirty patients (18 girls and 12 boys) with a mean age of 10.5±0.7 years were treated with the R-appliance for 16.2±0.3 months and 25 (11 boys and 14 girls) with a mean age of 11.2±1.3 years with a TB for 16.1±1.4 months (control). All had a Class II division 1 malocclusion due to mandibular deficiency. Lateral cephalograms obtained at the beginning (T1) and end (T2) of the study were analysed. Paired t-tests showed that SNB significantly increased in both groups. The incisor mandibular plane angle (IMPA) was reduced in the R-appliance group by 1.9±4.9 degrees (P<0.04) but increased by 0.5±5.1 degrees (P<0.6) in the TB group. SNA in the R-appliance group showed an increase of 0.2±1.8 degrees (P<0.5), while it was decreased by 0.2±1.3 degrees (P<0.3) in the TB group. Both treatment modalities were successful in moving the mandible forward. However, with the R-appliance, this was achieved without retroclination of the lower incisors.