We examined the effect of nedocromil sodium on histamine airway responsiveness in twelve grass-pollen sensitive patients during the 1984 pollen season. The study was a randomized double-blind crossover comparison of nedocromil sodium administered by a pressurized aerosol (4 mg b.d.) with placebo. Crossovers were made at 14-day intervals throughout 8 weeks of the grass pollen season. Histamine airway responsiveness was assessed twice before the pollen season and at the end of each 14-day treatment period. Results were expressed as the provocation concentration (PC) producing a 10% fall in FEV1 (PC10 FEV1) and a 40% fall in flow at 30% of the vital capacity (PC40V̇30(P)). During the pollen season all patients developed hay fever and seven had symptoms of asthma. The observed lowest values of PC10 FEV1 and PC40V̇30(P) during the placebo treatment periods were significantly lower than mean preseasonal values although not significantly lower than theoretical expected values. Geometric means PC10 FEV1 and PC40V̇30(P) were significantly higher during nedocromil sodium treatment compared with placebo. These results indicate that nedocromil sodium has a small but statistically significant effect reducing histamine airway responsiveness in grass-pollen sensitive patients during the pollen season.