In this retrospective investigation, the changes occurring during the treatment of patients with posterior crossbite in the mixed dentition with the use of expansion plate and quad-helix appliances were evaluated and compared with those resulting from growth and development occurring in a control group of patients of similar age and type of malocclusion. The expansion plate group consisted of 13 patients, the quad-helix group of 14 patients, and the control group consisted of 10 children with transverse posterior crossbites in the mixed dentition. The research material was formed from orthodontic models and lateral and frontal cephalometric radiographs from 37 children. It was observed in this investigation that transverse expansion is achieved by both the expansion plate and quad-helix appliances. However, the average period of treatment was 1.2 years for the expansion plate, and 0.6 years for the quad-helix appliance. Although posterior crossbite was corrected in a fairly short period of time, the quad helix appliance caused considerable buccal tipping of the maxillary first permanent molars.