Purpose The goal was to study the effects of early cervical headgear treatment on maxillary and mandibular dental arch area, shape and interarch dimensions. Methods The total study group comprised 67 children aged 7.6 years (standard deviation 0.3) with Angle class II malocclusion collected between 1992 and 1996. The children were randomly divided into two groups of equal size. In the first group, cervical headgear treatment was started immediately and undertaken for 2 years. The remaining patients served as untreated controls. Dental casts were taken and scanned at the beginning of treatment (T0) and at the 2‑year (T1) and 4‑year follow-up (T2). Three-dimensional landmarks describing the positions of maxillary and mandibular incisors, canines, first and second premolars and first molars were used to calculate and visualize the maxillary and mandibular dental arch area and shape using the polynomial equation y = Ax6 + Bx2. Results Significant changes in the shape and area of both maxillary and mandibular dental arches were induced with cervical headgear. The headgear increased dental arch area, sagittal dimensions at the mid-sagittal line and transversal dimensions at all of the measured levels in both dental arches compared to the control group. Conclusions Cervical headgear is an effective treatment device to gain space in both dental arches. Furthermore, when used as an early phase treatment, relapse is relatively small compared to the gained space.