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This study was carried out to determine whether the minimal important difference, in evaluative quality of life instruments which use a 7-point scale, is similar across individual domains and for both improvement and deterioration. Thirty nine adults with asthma were studied, using an 8 week cohort with assessments at 0, 4 and 8 weeks. The outcomes were the Asthma Quality of Life Questionnaire and global rating of change. For overall asthma-specific quality of life and for all individual domains (activities, emotions, symptoms), the minimal important difference of quality of life score per item was very close to 0.5 (range: 0.42–0.58); differences of approximately 1.0 represented a moderate change (range: 0.77–1.51); differences greater than 1.5 represented large changes. Changes for improvement and deterioration were very similar. The changes in quality of life score that represent a minimal important difference are …