Nasal mucociliary clearance (NMC) time and nasal mucus pH were studied in 50 patients suffering from diabetes mellitus and in a group of 50 healthy non-smokers and non-alcoholic controls. NMC time and pH values were found to be significantly increased in diabetics (NMC = 18.02 +/- 5.08 and pH = 7.96 +/- 0.75) as compared to controls (NMC = 7.49 +/- 1.06 and pH 6.43 +/- 0.67). The increase in NMC and pH was much more in patients having insulin dependent diabetes mellitus (IDDM) (NMC - 20.87 +/- 4.71 and pH -8.38 +/- 0.56) than in non-insulin dependent diabetes mellitus (NIDDM) (NMC - 15.16 +/- 3.67 and pH -7.53 +/- 0.687) and also when the duration of disease was more than 10 yr (NMC - 22.36 +/- 4.36 and pH -8.47 +/- 0.607). This impairment was attributed to osmotic diuresis with loss of water and electrolytes from all parts of the body and also small vessel abnormalities encountered in diabetes.