The efficacy and safety of cromolyn sodium by metered-dose inhaler (MDI) (1 mg per actuation) was evaluated with a double-blind, placebo-controlled, parallel-study design. Subjects with asthma, aged 8 to 58 years, whose asthma was well controlled taking cromolyn sodium capsules by Spinhaler turboinhaler, plus β2-agonists, entered the study after being maintained with cromolyn sodium capsules for a minimum of 4 weeks. The investigation began with a 2-week control interval with cromolyn sodium capsules followed by a 4-week single-blind period with placebo capsules. Subjects whose asthma significantly worsened while they were receiving placebo therapy were then randomized to a 10-week double-blind phase in which they received either active cromolyn sodium or placebo by MDI. Efficacy variables included diary data, physician evaluation, and spirometry. Comparisons were made between baseline period scores and each assessment variable over time. Of 155 subjects entered, 93 qualified for the double-blind, randomized phase. Eighty-three subjects completed the study and were analyzed At baseline there existed no significant differences between the active-treatment and placebo-treatment groups. Significant differences (p < 0.05) in favor of the cromolyn sodium-treatment group, however, were noted at all time points for daily diary symptoms (cough, breathlessness, and overall asthma severity), physician's assessments at each clinic visit, physician's and patient's overall final assessments, FEV1 at each clinic visit, and FVC and peak expiratory flow rate at the final visit. Concomitant bronchodilator medication use was less in the cromolyn sodium-treatment group. Cromolyn sodium by MDI is highly effective for (1) controlling asthmatic symptoms, (2) improving lung functions, and (3) decreasing the need for concomitant bronchodilators.