There has been a long-standing controversy over the efficacy of rapid maxillary expansion to relieve nasal obstruction and improve respiration. Recently rhinomanometry has provided a discipline for the investigation into nasal airway physiology with quantifiable parameters for evaluation and comparable studies. In this trial, a sample of 26 patients (13 male and 13 female, age range 10.10 to 19.6 years), receiving rapid maxillary expansion as part of their orthodontic mechano-therapy, were appraised for nasal airway resistance before and after expansion. The posterior rhinomanometric technique was used, measuring the respiratory flow between pharynx and the nostrils at a preset pressure difference between these two points. The formula for calculating the resistance is derived from the electrical Ohm's Law and requires that the pressure difference be divided by the flow. Reductions were recorded in all cases with an average of 36.2 per cent (range 11.6-58.6). The correlation between the resistance reductions and the delivered expansions (increases in trans-palatal widths) was weak (r = 0.32). In view of the probable significance of the liminal valve in nasal resistance, expansions in this area were assessed by changes in the trans-alar widths. The correlation between trans-alar increases and the trans-palatal expansions was weak (r = 0.115), as it was between the trans-alar increases and the reductions in nasal airway resistance (r = 0.30).