The quad-helix appliance is a fixed lingual arch wire appliance that produces slow maxillary expansion for the treatment of maxillary constriction or crossbite in the primary and mixed dentitions. Despite considerable success in the clinical treatment of patients, there is little quantitative information about the force delivery of this appliance. The purpose of this investigation was to characterize the in vitro force delivery of the quad-helix appliance. Appliances of four sizes were fabricated from Elgiloy blue and stainless steel wires with diameters of 0.032, 0.036, and 0.038 inch. The force delivery in grams for 8-mm activation was determined from the force-extension characteristics using an Instron mechanical testing machine. The results show that the force delivery of the quad-helix appliance is affected significantly by changes in the appliance size and arch wire diameter, but is independent of alloy type (stainless steel or Elgiloy blue). The quantitative results from this investigation should provide worthwhile information for clinical appliance selection and treatment of patients with varying arch dimensions.