The aim of this study was to relate the initial orthopaedic effects of a Bass activator to age and growth periods at the time of therapy. The sample consisted of 54 boys with a severe Class II, division 1 malocclusion. The mean age at start of treatment was 11.4 years. Longitudinal records of standing height were used to assess the growth period during treatment. Three periods were established: pre-peak, peak and post-peak. The treatment period of each subject was assigned to one of these periods. Lateral cephalograms were available 6 months before treatment in 22 of the boys, and at the start of treatment and after 6 months in the whole sample. No significant growth changes were observed 6 months before treatment. During the initial 6 months of treatment a small restraining effect on the maxilla and a forward growth of the mandible was observed. A significant correlation between age and forward growth of the mandible during treatment was found in the pre-peak and peak groups. This indicates a great initial orthopaedic treatment effect in young boys in the pre-peak period and in boys with an early peak. In boys 10 years of age or older not having reached the peak period of growth, the treatment response was less pronounced.