The effect of the tonicity of saline nasal douching solutions on mucociliary clearance was studied in order to ascertain whether hypertonicity conferred any advantage. Thirty-eight normal subjects were included in a randomised double-blind crossover trial. Saline douching solutions of 0.9%, 3% and 5% tonicity were used and mucociliary clearance was measured by the saccharin clearance time (SCT). The resultant SCT after administration of 5% saline was significantly reduced compared to both 0.9% (P = 0.005) and 3% saline (P = 0.04). There was no difference between 0.9% and 3% saline administration. Thus hypertonic saline solutions improve mucociliary clearance, although this was only observed with solutions of 5% tonicity. The effect is probably brought about by changes in mucus rheology.