The purpose of this study was to evaluate the effect of rapid maxillary expansion (RME) on nasal minimum cross-sectional area (MCA) using acoustic rhinometry (AR) in two groups of subjects who were treated before and after the pubertal growth spurt. The sample consisted of 29 patients with maxillary constriction and a control sample of 15 subjects. Both samples were divided into two groups according to individual skeletal maturation as assessed by the cervical vertebral maturation (CVM) method. Group I T (early-treated) consisted of 16 patients (eight girls and eight boys). Group I C (early-control) consisted of eight patients, and both groups had not reached the pubertal peak (CVM Stage 1-3). Group II T (latetreated) consisted of 13 patients (eight girls and five boys). Group II C (late-control) consisted of seven patients, and both groups were at a stage during or after the pubertal peak (CVM Stage 4-6). AR records were obtained for each treated subject before treatment (T-1), after expansion (T-2), and immediately after a three-month retention period (T-3); only T-1 and T-3 records were obtained for controls. The overall increase in MCA was significantly greater in the early- and late-treated groups (group I T, group II T) as compared with the early and late controls. (group I C, group II C) (P <.05). The results of the present study suggest that even the overall (T-1-T-3) increase for MCA in group I T is greater (0.34 mm) than the increase for MCA in group II T (0.19 mm), but the difference was not significant (P >.05).