Two methods of defining perception of asthma-related changes in airflow were compared, and relationships to clinical opinions of severity and assessments of psychological functioning were investigated. Perceived breathlessness (VAB) and peak expiratory flow (PEF) were recorded by 100 subjects for 28 days. Perception was defined by correlation of the two values and by the ratio of maximum change in PEF and related change in VAB. The latter method defined 24 poor perceivers (PP) and 13 exaggerated perceivers (EP), in whom presence of a psychological disorder was high (30% of PP, 54% of EP, compared to 8% of the remainder). Clinical severity in EP was assessed as greater than appeared warranted.