This is a pilot study on effects of cromolyn on asthmatic subjects in Tehran, which is a randomized double-blind and placebo-controlled clinical trial. Among asthmatic patients who referring to Department of Internal Medicine of Imam-Khomeini Hospital, 40 patients with a documented diagnosis of bronchial asthma with a FEV1 < 80% of predicted value, and reversible airflow obstruction which are in a stable condition, enter to the study. Exclusion criteria include acute asthma attack, smoking, respiratory tract infections within 1 month of the study (according to history and physical examination) and presence of chronic diseases such as heart disease. After announcement of study, volunteers would be attended the laboratory at two days. In the day one, firstly for ethic consideration, the consent would be obtained in writing from each volunteer after that a brief explanation of trial including efficacy and safety of drug in asthma was given. Then subjects were instructed on how to perform spirometric test and on the technique for inhaling sodium cromolyn through the spinhaler, by an expert technician. Also, method of antiasthma drug discontinuation would be ordered to patient, by a physician as following: β2-adrenoceptor agonist would be withheld for 6h before the study, methylxanthines and cromolyn would be withheld for 24 h, and aerosol steroids would be withheld on the morning of the study day.Thereafter, patient would be entered by random allocation to the following groups: inhalation of cromolyn-20mg, inhalation of cromolyn-40mg or inhalation of cromolyn-placebo In the second day (two days later), on arrival at the laboratory after a rest time, the subject would perform three lung function maneuvers. The best value would be taken as baseline spirometry. Then for determining effect of cromolyn, lung function tests would be repeated at 15, 30 and 60 minutes after inhalation. Then at each time, percent of improvement in important lung function variables, such as forced expiratory volume in one second (FEV1) from baseline values would be calculated and compared between three groups. One hour after the last test, patient will be observed for any adverse effect of drug