12 children with known bronchial hyperreactivity and asthma had isocapnic cold air hyperventilation challenges of their bronchial airways. In a double-blind cross-over study they inhaled either placebo or nedocromil sodium (2 puffs à 2 mg) from a MDI before challenge. 2 subjects were found to be non-responders during the two days of the study. The remainder all reacted on the placebo days, while 5 subjects were completely protected on the nedocromil-days and 5 subjects were partially protected. Nedocromil is a potent agent for the suppression of bronchial hyperresponsiveness.