In a multicenter, double-blind, group comparative trial, the efficacy of nedocromil sodium (nedocromil, 4 mg, four times daily [qid]), cromolyn sodium (2 mg, qid), and placebo was compared in patients receiving inhaled beta 2-agonists and inhaled corticosteroids for the treatment of chronic reversible obstructive airway disease. After a 2-week baseline period, 132 patients (8 centers) between the ages of 20 and 75 years entered a 4-week run-in period in which the dose of inhaled corticosteroid was reduced by 50 percent. During the run-in phase, deterioration of symptoms (total symptom score) by ten points qualified patients to enter the 6-week drug trial period. Patients in the nedocromil treatment group showed the most robust and consistent improvements over placebo and cromolyn sodium for all daily dairy variables. Statistically significant improvements over placebo were noted for both active treatment groups for daytime, nighttime, and total symptom score. Symptom scores for nedocromil were statistically significantly improved over both cromolyn sodium and placebo for both daytime and nighttime asthma. Patients treated with nedocromil also demonstrated a significant reduction in the use of nighttime as needed (prn) beta 2-agonists as compared with either the placebo- or cromolyn sodium-treated groups. Only nedocromil-treated patients demonstrated a statistically significant improvement in morning peak expiratory flow rate (PEFR) as compared with placebo. Both nedocromil and cromolyn sodium groups demonstrated statistically significant improvements in afternoon and evening PEFRs. Collectively, the improvements in nighttime symptoms, decreased bronchodilator use, and improved morning PEFR show that patients treated with nedocromil had improved nocturnal symptoms. Pulmonary function tests (FEV1, FVC, PEFR) demonstrated no statistically significant differences between the two active treatments, although trends favored nedocromil for both FEV1 and PEFR. Although symptoms improved in patients treated with cromolyn sodium, the level of symptom control was less than that achieved by nedocromil. As compared with baseline control (regular dose of inhaled steroids), patients treated with nedocromil plus the 50 percent reduced dosage of inhaled corticosteroid consistently demonstrated comparable or better symptom control. Although both active drugs reduced symptoms, nedocromil proved to be more effective than cromolyn sodium for treatment of reversible obstructive airway disease in patients normally well maintained on regimens of low to moderate doses of inhaled corticosteroids.