Clinical and case control methods were used to identify the role of irritant exposure in adult-onset asthma in a community-based prospective study of asthma incidence. The study cohort was a typical US working population enrolled in an HMO. Additional benefits of the study design were the opportunities to validate a questionnaire for exposure assessment and for detection of work-related asthma. Specifically, the study: 1) Investigated incident cases in a cohort of over 80,000 adults over three years and determined the proportion that met a clinical definition of occupational asthma (OA); 2) Used a nested case-control study to determine the incidence of all asthma by occupation and workplace exposure; 3) Determined whether clinical OA accounted for the excess incidence of adult-onset asthma associated with workplace exposure to sensitizers and irritants; 4) Prospectively followed asthmatics for two years after diagnosis to determine the impact of adult-onset asthma on lung function, employment, income, and quality of life, and to determine whether prognosis differed for clinical OA and for asthma associated with workplace irritant exposure; 5) Tested an intervention designed to increase appropriate clinical diagnosis, and thus secondary prevention of OA.