Health-related quality of life (HRQL) has become an important outcome in asthma, since traditional outcomes, such as respiratory symptoms and pulmonary function, might not entirely express the patient’s perception of the limitations caused by the disease. The aim of this thesis was to study HRQL in asthma and to analyse if HRQL was related to asthma onset and prognosis. Other aims were to identify determinants of low HRQL in clinically-verified asthmatics, and to study whether low HRQL was a predictor of mortality. In 1990, a self-administered questionnaire was completed by 12,560 individuals from three age groups (16, 30-39, and 60-69 years) in two counties of Sweden. In a second phase, all subjects who reported a history of obstructive respiratory symptoms (n = 1,851) and 600 randomly-selected controls were invited to a clinical investigation including spirometry, allergy testing, and assessment of HRQL with the Gothenburg Quality of Life instrument. In 2003, the eligible subjects in the cohort (n=11,282) were sent a new questionnaire. Mortality data in the cohort was followed up during 1990–2008 using data from the National Board of Health and Welfare Mortality Database. The 616 subjects with clinically-verified asthma 1990 had significantly lower HRQL than subjects without asthma. In the 2003 follow-up, the 305 subjects with persistent asthma had a lower HRQL than the 155 subjects who showed improvement in asthma during the follow-up. Subjects who had developed asthma by the follow-up had a significantly lower HRQL at baseline than those who did not develop asthma. Significant determinants of quality of life in asthma were female sex, smoking habits, higher airway responsiveness to irritants, respiratory symptom severity, positive skin prick test, and absenteeism from work or school. Low HRQL was related to increased mortality, but this association was not found when analyzing the asthmatic group alone. In conclusion, measurements of HRQL are of value for evaluating both the impact and progression of asthma.