We evaluated the prevalence of asthma and its predictors in studies of several male working groups: 619 cedar sawmill, 724 grain elevator, 399 pulpmill, 798 aluminum smelter, and 1,127 unexposed workers. These workers had taken part in health studies for assessment of chronic respiratory effects of various workplace exposures between 1979 and 1982. The American Thoracic Society Adult Questionnaire (ATS-DLD-78) was used for these studies. Allergy skin tests were also performed. The participation rates were > 80%. The overall prevalance of physician-diagnosed asthma was 4.6%, and current asthma 3%. The prevalence of asthma after employment in the current industry, as a surrogate for work-related asthma, was 3.9 times higher in cedar sawmill workers, 2.2 times higher in pulpmill and aluminum smelter workers, and 1.7 times higher in grain elevator workers compared with unexposed workers. Atopy and a positive parental history of asthma, but not smoking, were important risk factors for asthma before the onset of first employment. Also, for asthma after employment in the current industry, atopy and a positive parental history of asthma were important risk factors. Smoking was associated with a significant reduction in the risk for asthma after employment in the current industry. Within specific work groups, the prevalence of atopy was significantly higher among pulpmill workers with asthma after employment in current industry than those without asthma. Conversely, cedar sawmill workers who had asthma after employment in the current industry were nonatopic and nonsmokers.