The efficacy of nedocromil sodium (NED) (4 mg twice daily) and beclomethasone dipropionate (BDP) (200 micrograms twice daily) in controlling the symptoms of asthma, the pulmonary function, and bronchial responsiveness to histamine was assessed in a double-blind, double-dummy, crossover study of 39 adult patients with chronic asthma. The patients, most of whom were assessed to be affected to a moderate degree, were insufficiently controlled in their current regimen of inhaled and/or oral bronchodilators. A 2-week baseline period preceded 6 weeks of treatment with each of the study drugs. Both treatment groups demonstrated improvements from baseline in clinical assessment of lung function performed after the first 6 weeks of treatment. No significant differences were observed when the effects of the treatments were compared on FEV1, FVC, and peak expiratory flow. Bronchial reactivity to histamine, measured as the amount of histamine causing a 20% fall in FEV1 (PC20), decreased significantly (p less than 0.05) after 6 weeks of treatment with BDP compared to the effect of NED treatment. Asthma severity, symptom score, and inhaled bronchodilator use demonstrated significantly better results with BDP. After crossover of treatment, the group transferring from NED to BDP continued to improve, whereas the group crossing from BDP to NED tended to demonstrate a major deterioration during the first 3 weeks, after which a stabilization or an increase in FEV1, FVC, and ln PC20 appeared to occur. It is concluded that NED for inhalation is a potent new drug for treatment of both atopic and nonatopic subjects with asthma. With the number of patients and dosages used, the effect on the pulmonary function was not significantly different from that of BDP after the initial 6 weeks of treatment, but BDP had a better effect on asthma severity, overall opinions, symptom score, bronchodilator use, ln PC20, and morning peak expiratory flow.