Aim: The development of a class II malocclusion is usually related to a reduced oropharyngeal airway space. In order to prevent airway obstruction, functional appliances are commonly used for orthodontic therapy. The aim of the article is to verify if these appliances could positively influence oropharyngeal diameters preventing the onset of future respiratory disorders. Materials and methods: A group of 10 patients treated at the Dental Clinic of San Gerardo Hospital in Monza with mandibular activator was selected. Ten similar untreated class II subjects with retrognathic mandible were used as control group. The cephalometric tracings were made on lateral teleradiographs of the skull before and after the active therapy with functional devices for the treated group and before and after growth peak for the control one. Descriptive statistical analysis was calculated for all the cephalometric values in both study cases and control group using Excel worksheet. The data distribution was evaluated with Shapiro–Wilk test and the in-between group discrepancies were evaluated with Mann–Whitney U test. Results: At T1 period, both case and control groups showed a class II pattern. At T2 period, the study group shows an improvement in bones relationship with reduced ANB angles and OVJ measurements. The control group otherwise did not show any important changes in maxillo–mandibular discrepancies. The airway size increased in either group in most cases. No significant differences between the treated and control groups were detected for airway size neither in the upper, middle, nor lower level at the T1–T2 interval. Conclusion: The upper airway values did not show any significant discrepancies between the two groups during the observation period. Clinical significance: Functional devices were effective in solving class II relationships, but there is no evidence of successful breathing disorders prevention by using mandibular activators, probably due to the stability of airway tissues reached in pubertal age in both groups.