A 20-item self-administered questionnaire with Likert scale responses was developed to measure quality of life in adult subjects with asthma. A total scale score together with subscale scores for breathlessness, mood disturbance, social disruption and concerns for health were calculated by addition of item scores. Items for the scale were selected by principal components analysis of the responses of 283 subjects to a preliminary pool of 69 items. In 58 subjects with stable asthma, good short term test-retest reliability was demonstrated with the intraclass correlation coefficient for the total scale being 0.80. The questionnaire was internally consistent in a sample of outpatients (Cronbach's alpha 0.92 in 77 subjects) and in a community sample with asthma (Cronbach's alpha 0.94 in 87 subjects). Weak correlations in the expected direction were seen with three medical markers of asthma severity. This supports the construct validity of the questionnaire and emphasizes that quality of life represents a separate dimension of asthma.