A double-blind, placebo-controlled study was performed to determine the efficacy and safety of cromolyn sodium (Intal) administered to children by metered dose inhaler (MDI). Prior to entry, subjects were well controlled on cromolyn sodium capsules by Spinhaler turbo-inhaler plus beta 2 agonists. An active control interval of 2 weeks on cromolyn sodium capsules was followed by a 4-week single-blind period on placebo capsules. Those subjects whose asthma worsened significantly on placebo entered a 10-week double-blind phase, randomized to receive either cromolyn sodium (2 mg per dose) or placebo by MDI. Diary data, physician evaluation, and pulmonary function tests were used to assess efficacy, and scores were compared with the baseline value at 2-week intervals. Forty children with asthma, 8 to 20 years of age, entered the study and 32 qualified for the randomized phase. No significant differences existed between the treatment groups at baseline. Most comparative data favored the cromolyn sodium group over the course of the study. Significant differences (p less than .05) were noted for diary scores of breathlessness and overall asthma severity. There was significant improvement at the final visit favoring the cromolyn sodium group in restriction on normal activity, FEV1, and PEFR. The cromolyn sodium group also experienced a decreasing need for concomitant bronchodilators. Both groups preferred pressurized aerosol by MDI over powdered capsules by Spinhaler. (Intal and Spinhaler are registered trademarks of Fisons Corporation.)