This prospective, randomized, blinded clinical trial was conducted to test whether therapy with cromolyn sodium might decrease the incidence or severity of bronchopulmonary dysplasia when given to newborns with respiratory distress syndrome. Cromolyn (20 mg) or placebo was aerosolized to intubated newborns with respiratory distress syndrome every 6 hours, beginning on the first day of intubation. Patients were stratified by birth weight less than 1000 g and 1000 to 2000 g; primary outcome success was defined as survival to 30 days without oxygen dependence. Of 10 patients enrolled who were less than 1000 g birth weight, there were no treatment successes, preventing outcome analysis. The study was discontinued after 28 patients of 1000 to 2000 g birth weight had been studied, at which time it had been found with 95% confidence, with a power of .80, that cromolyn sodium did not decrease by 50% the incidence of bronchopulmonary dysplasia. Severity of bronchopulmonary dysplasia was also similar, with 4 patients in the treatment group and 3 in the placebo group receiving mechanical ventilation at 30 days. Possible reasons for this study outcome include (1) a delivered dose too small to produce a clinical effect; (2) the start of therapy too late to prevent the onset of inflammation; (3) inadequate effect of cromolyn on polymorphonuclear cells in vivo; or (4) development of bronchopulmonary dysplasia through factors unaffected by the actions of cromolyn.