Background. Breathlessness in asthma often cannot be explained with objective variables indicating airways obstruction. The hypothesis that unrealistic breathlessness results from false interpretation of sensations was tested.

Methods. Sixty-four children and adolescents with asthma, aged 9–18 years, were randomly assigned to: (1) standardized physical exercise for induction of general symptoms; (2) equipment causing itching through skin irritation; or (3) physical exercise combined with equipment causing itching through skin irritation. Pre-test and post-test measures were: lung function; breathlessness; general symptoms; itching; state anxiety; and worry.

Results. Lung function decreased within normal parameters (3·8%, 1·1%, 2·6%, respectively) and did not differ significantly between conditions. Breathlessness increased significantly after exercise, particularly in condition 3. Breathlessness correlated with general symptoms and worrying, but not with changes in lung function, age, or asthma severity.

Conclusion. Biased symptom perception can explain unrealistic breathlessness. Prerequisites are situational cues triggering selective perception and ambiguous sensations associated with the anticipated (feared) physical state. Excessive breathlessness may often warrant objective confirmation by means of lung-function testing.