Twenty-three children with chronic severe perennial asthma received randomly-allocated disodium cromoglycate or placebo four times a day for 12 weeks, and the alternative regimen for the subsequent 12 weeks. More than half the patients improved while on DSCG according to clinical assessment. There was a significant increase in the mean FEV(0.75 second) during the drug period, largely owing to dramatic improvement in nine patients. No reduction in the mean decrease of FEV after exercise was demonstrated. Response, when it occurred, was evident within four weeks. The effect of the medication was consistent in individual patients throughout the 12-week period. No evidence of toxicity was discovered during the period of study.