There are few cost evaluation studies of orthodontic treatment. The aim of this study was to determine the costs of correcting posterior crossbites with Quad Helix (QH) or expansion plates (EPs) and to relate the costs to the effects. To determine which alternative has the lower cost, a cost-minimization analysis was undertaken, based on that the outcome of the treatment alternatives is identical. The study comprised 40 subjects in the mixed dentition, who had undergone treatment for unilateral posterior crossbite: 20 with QH and 20 with EPs. Duration of treatment, number of appointments, broken appointments, and cancellations were registered. Direct costs (for the premises, staff salaries, material and laboratory costs) and indirect costs (loss of income due to parent's assumed absence from work) were calculated and evaluated for successful treatment alone, for successful and unsuccessful treatment and re-treatment when required. The QH had significantly lower direct and indirect costs, with fewer failures requiring re-treatment. Even the costs for successful cases only were significantly lower in the QH than in the EP group. The results clearly show that in terms of cost-minimization, QH is the preferred method for correcting posterior crossbite in the mixed dentition.