In this double-blind placebo controlled crossover study the protective effect of nedocromil sodium against bronchial allergen challenge was evaluated in ten asthmatic patients. Single doses of 0.5, 1.0 and 2.0 mg nedocromil sodium, administered as a pressurized aerosol 3 hr prior to challenge, were each significantly more effective than placebo in inhibiting allergen-induced bronchoconstriction. The 1.0 and 2.0 mg doses of the active drug were significantly more effective than the 0.5 mg dose. No significant difference was demonstrated between the 1.0 and 2.0 mg doses. Duration of protection beyond 3 hr was not tested. The results suggest that nedocromil sodium may be a potentially useful therapeutic agent and is worthy of further evaluation for the treatment of reversible obstructive airways disease.