OBJECTIVE: Concepts of quality of life are gaining increasing significance as measures of outcome, both in medicine as well as in economics. Therefore - particularly for chronic illnesses such as asthma - instruments are necessary which make the influence of the illness on the daily life of the patients measurable from a subjective point of view. One such questionnaire is the FLA ("Fragebogen zur Asthmaqualität"), a disease-specific instrument for measuring the quality of life of asthma patients. This questionnaire was supplemented with a generic instrument, the EuroQol. The aim of the study was to measure the quality of life depending upon the severity level of the illness. METHOD: Adult asthma patients treated in 23 pneumologic ambulatory care units were randomly selected and asked to fill out both the FLA and the EuroQol questionnaires. Separate analyses of the results were carried out for each patient severity level, and covered the average values of the subscales, the correlation as well as the internal reliability of the questionnaires. RESULTS: The quality of life of asthma patients correlates strongly to the respective severity level of the illness. This applies to all the examined dimensions of health-related quality of life. The patients of level 3 (serious asthma) in particular achieve values far below average when compared to the total population (42 out of 100 according to EuroQol). By contrast, the subjective quality of life of asthmatics in level 1 is only slightly reduced. The correlation of the FLA subscales as well as of the EuroQol values is high (0.74 - 0.97). CONCLUSIONS: The study makes clear that a progression of the illness is associated with high losses of quality of life for the patient. From a health-economic point of view this is important for the calculation of so-called intangible costs, which are often not quantified in studies on the cost of disease.