Introduction: Functional posterior crossbite (FPXB) malocclusion is frequently seen in the deciduous or mixed dentition. It is often accompanied by lateral mandibular shift and mandibular midline deviation because of the reduction in the width of the maxillary dental arch. The aims of this prospective study were to examine in detail the morphologic, skeletal, dental, and functional effects of FPXB, and the effects of maxillary expansion treatment with quad-helix appliance. Methods: The experimental group consisted of 35 FPXB patients (20 girls, 15 boys) having a mean age of 10.6 +/-_ 1.4 years; the control group consisted of 31 normocclusive subjects (18 girls, 13 boys) with a mean age of 9.8 +/- 1.6 years. Lateral, posteroanterior, and submentovertex cephalograms, transcranial temporomandibular joint radiographs, joint vibration analysis, and electromyographic recordings were obtained from every patient before and after maxillary expansion. Magnetic resonance images were taken before treatment for diagnostic purposes. These data were collected at 1 time point in the controls. Results: The pretreatment posteroanterior, submentovertex, and transcranial temporomandibular joint radiographs showed mandibular asymmetry relative to the cranial base and condylar malpositioning in the glenoid fossa. Joint vibration analysis findings showed different vibrations between the crossbite and noncrossbite sides, and imbalanced electromyographic findings in the experimental group. After treatment, the asymmetric morphology and position of the mandible and condyles were eliminated, and the stornatognathic system functions were normalized. Conclusions: Early orthodontic treatment of FPXB creates optimum conditions for normal growth of the craniofacial skeleton and normal function of the stornatognathic system.