Sagittal intermaxillary changes brought about by functional appliances show large inter-individual variation. One factor that may in part explain these differences is the masticatory musculature and its functional capacity. The aims of this study were to investigate changes in maximal molar bite force during functional appliance treatment and to assess the influence of pre-treatment maximal molar bite force on treatment outcomes with functional appliances used in Class II malocclusion children. Twenty-five children (17 males and 8 females), aged 9-13 years, with a Class II malocclusion and increased overjet were treated with functional appliances for 1-2 years. Dental casts, lateral cephalograms, maximal molar bite force, and finger force measurements were performed before (T1) and after (T2) treatment. These same measurements were also performed 1-2 years before treatment (T0); the intermediate period before starting treatment served as the control. Multiple regression analyses were used to determine possible correlations between initial maximal molar bite force and dental or cephalometric changes during treatment. Maximal molar bite force, which increased pre-treatment (T0-T1), decreased during functional appliance treatment (T1-T2). Children with a weaker T1 maximal molar bite force showed a larger overjet reduction, greater improvement in molar relationship, greater reduction in ANB angle, and greater augmentation in SNB angle from T1 to T2. Treatment of children with Class II malocclusions with functional appliances seems to lead to more favourable treatment outcomes in those with a weaker maximal molar bite force. This was observed both as regards improvements in dental sagittal relationships, namely overjet and molar Class, as well as skeletal changes due to a decrease in ANB and an increase in SNB angles.