Forty-one patients with an abnormal tear film break-up time entered this study. The effect, on the break-up time, of eyedrops containing 2% sodium cromoglycate was compared with placebo eyedrops. The tear film break-up time was prolonged by both treatments but the increase after sodium cromoglycate was much greater and was statistically significant. The effect of sodium cromoglycate was greater in females than males. These results can be explained by the behavior of sodium cromoglycate molecules in aqueous solution and by the increased viscosity of sodium cromoglycate solutions in the preserver of magnesium ions.