The study comprises an analysis of the effect of treatment with a modified activator combined with a high-pull headgear during a standardized observation period of the initial 6 months of treatment. All patients (24 girls and 32 boys, aged 8.5 to 15 years) had severe skeletal Class II malocclusion. In the first part of the study, the effect of treatment with the appliance both day and night is compared with the effect when it was worn only at night. Only a small and nonsignificant difference was found, but the patients tended to cooperate better if they were instructed to wear the appliance continuously. In the second part of the study, the effect of treatment is related to the somatic maturation of the patients. Longitudinal records of standing height were used to assess whether treatment had been performed before maximal pubertal growth (prepeak period), during maximal pubertal growth (peak period), or after maximal pubertal growth (postpeak period). The skeletal effect was significantly greater in boys treated during the peak period than in those treated during the prepeak period and a similar tendency, although not significant, was found among girls. The number of patients treated during the postpeak period was too small for statistical analysis.