Little is known about factors determining the outcome of childhood asthma. The purpose of this longitudinal study was to assess the factors in childhood that determine the level of FEV(1) in early adulthood in asthmatic individuals, and to examine factors associated with decline in FEV(1) during adulthood. Between 1966 and 1969, 119 allergic asthmatic subjects aged 5 to 14 yr were studied (Visit 1). Of these subjects, 101 (85%) were reinvestigated at ages 22 to 32 yr (Visit 2) and 32 to 42 yr (Visit 3). At the first survey and during follow-up, a standardized questionnaire was used, serum total IgE and peripheral blood eosinophils were measured, and physical examination, skin tests, lung function tests, and histamine challenge (provocative concentration causing a 10% decline in FEV(1); PC(10)) tests were performed according to the same protocol. Multiple linear regression analyses were performed with FEV(1) at Visit 2 and with the change of FEV(1) from Visit 2 to Visit 3 as outcome variables. A low FEV(1)% predicted at Visit 1 and PC(10) </= 16 mg/ml at Visit 1 were significantly associated with a lower level of FEV(1) at Visit 2. Subjects who quit smoking and subjects who continued to use inhaled corticosteroids had a significantly smaller annual decline in FEV(1) from Visit 2 to Visit 3, adjusted for attained level of FEV(1) at Visit 2. In conclusion, bronchial hyperresponsiveness and a low level of lung function in childhood are independent risk factors for a low level of FEV(1) in early adulthood. A smaller decline in FEV(1) after ages 22 to 32 yr occurs in asthmatics who quit smoking and who continue to use inhaled corticosteroids. Our data stress the importance of studying intervention strategies for asthma in young childhood and early adulthood in order to prevent or postpone further lung function deficits.