SummaryIn 23 children with asthma and 18 healthy controls, cold air challenge (CACh) was done twice during the same half day, and in the asthmatics a third time together with a histamine challenge (HCh) 2 wk later. Pulmonary functions were tested before and after each challenge. No overlapping of individual responses to CACh in seven forced expiratory flow tests proved the power of discrimination of this technique in children. The limits of “normal” reactions ranged from minus 9% for larger airway-related to 26% for smaller airway-jelated flows. Short-term reproducibility of induced changes, in percentage of baseline, was excellent (r=0.815-0.954); in percentage of predicted postchallenge abnormality it was even bettern (r=0.926-0.975). The response in small airway-related flow rates (-43.1 ± 12.8 to -51.9 ± 16.8% of baseline) was much larger than in others (-27.6 ± 14.6 to -32.1 ± 17.3% of baseline). This, the different baseline-toresponse correlations in various measurements, and the divergent dose response to colder versus less cold air in large (60.7 ± 21.9 versus 65.4 ± 21.5% predicted, postchallenge values) and small airway-related tests (28.9 ± 18.7 versus 29.5 ± 15.1% predicted, postchallenge values) in asthmatic children suggest a predetermined, small airway-related limitation of individual reactivity, which is independent of the baseline situation. All asthmatics responded positively to HCh but quantitative results of the two methods did not correlate. Responses to CACh also better characterized the clinical severity of asthma than those to HCh. Determining the individual optimum by a bronchodilator and the physiologic abnormality by CACh, the whole functional dimension of a child's asthma can be established.