Cromolyn sodium is a new, inhaled medication useful in the control of severe chronic asthma in children. Its mode of action is prophylactic, ie, it must be inhaled before events precipitating the asthma attack occur. In an initial eight-week double-blind crossover study against placebo in severely asthmatic children, improvement was found with use of the active drug in a majority. These patients were used to begin a long-term "open" study on cromolyn, which continued effective in mitigating asthmatic attacks in almost 50% of cases. Another 25% to 30% were able to reduce their intake of other medication. In addition, there was an overall improvement in exercise tolerance and a reduction in the incidence of exercise-induced bronchospasm. In keeping with other reports, we found that 20% to 25% of the patients failed to respond.