Abstract This article compares the arch development in growing Class II patients after Fränkel therapy with arch development in a similar group of untreated Class II subjects. Sixty treated and forty-seven untreated persons were compared for a 2- to 4-year period. Expansion of the maxillary and mandibular dental arches occurred to a much greater extent in patients undergoing Fränkel treatment than in the control subjects. The expansion was not limited to a particular region of the dental arch in the treated cases, although the largest expansion values were recorded in the premolar and molar regions. Lesser values were recorded in the canine region.