In this study, the effects of cervical headgear (CHG) use on the transverse dimension of the maxillary dental arch were evaluated in patients in the permanent dentition. Thirteen girls and 12 boys (mean age: 13.41 +/- 0.52 years) with a bilateral full cusp Class II molar relationship comprised the study group. Fifteen girls and 10 boys with a Class I normal occlusion comprised the controls. In the treatment group, CHG with an expanded inner bow was used for a mean period of 11.2 +/- 5.6 months. The headgear was used for molar distalization and the force magnitude was 196.1 cN. After CHG treatment, the patients underwent non-extraction fixed orthodontic treatment for 14.1 +/- 2.5 months. During this period, the control group received regular dental check-ups. Dental casts obtained at the beginning (T1) and end (T2) of headgear use and at the end of orthodontic treatment (T3) and posteroanterior cephalograms taken at T1 and T2 were evaluated. A Student's t-test was used for intergroup comparison at T1, T2, and T3 and a Mann-Whitney U-test with a Bonferroni correction for comparison of treatment/observation changes.