The aim of this study was to investigate whether nasal prongs, which have been proposed to assess nasal flow during sleep, affect nasal airflow resistance (NR).NR was estimated by posterior rhinomanometry at a 0.5 L/s flow, under eight conditions: in the basal state, and with seven different nasal prongs.The study was performed in 17 healthy supine subjects, 8 of whom had basal NR values within the normal range ( 2.5 cm H(2)O.L(-1).s, group 2), because of nare narrowness and/or deviated nasal septum.NR increased significantly while breathing with nasal prongs (p < 0.0001 in both groups). The changes in NR (DeltaNR) induced by the different nasal prongs were characterized by large intersubject and intrasubject variability, with a maximum DeltaNR of 24.2 cm H(2)O.L(-1).s. Significant differences were found between the DeltaNR induced by the different nasal prongs (p < 0.001 in group 1, and p < 0.0003 in group 2), and for six of them, DeltaNR was significantly higher in group 1 than in group 2 (p < 0.02).This study demonstrates that nasal prongs can markedly increase NR in subjects presenting with nare narrowness and/or deviated nasal septum. Further investigations that would include nocturnal polysomnography are still required to evaluate the possible influence of nasal prongs on the diagnosis of obstructive sleep apnea syndrome and its severity.