The aim of this study was to investigate the effects of the headgear-Herbst appliance with mandibular step-by-step advancement followed by retention with the headgear activator. The sample comprised 22 consecutively treated patients (13.2 +/- 1.5 years old) and 31 matched controls (12.6 +/- 1.3 years old). Lateral cephalograms were obtained at the beginning of treatment, after 6 months (at completion of the initial phase), after 12 months of active treatment (at the end of the late phase), and after 18 months (at the end of retention). The results showed that the skeletal effects over 12 months of treatment were restrained maxillary growth, enhanced mandibular growth, and reduced increase in lower facial height. The restraint effect on the maxilla was significant in both the initial and late phases; the enhanced growth of the mandible was significant during the initial phase only; and the height of the lower face was affected during the late phase only. During retention with the headgear activator, the jaw base relationship was maintained, and the effect on the lower facial height was reinforced. A small relapse of the overjet and molar relationship was the result of dental changes only. During 18 months of active treatment and retention, the overjet correction was the result of 70% skeletal changes. The headgear-Herbst appliance is an effective orthopedic device, and the effect on the maxilla seems to increase with the length of treatment, but the pattern was reversed for the mandible.