The prevalences of acute and chronic respiratory symptoms and lung function changes were studied in a group of 806 workers working in a "clean environment." Acute symptoms during the work shift were reported by smokers but were essentially absent in nonsmokers. Chronic respiratory-symptom prevalences were similar to those reported in community-based populations, with smokers experiencing more chronic symptoms than nonsmokers. Baseline lung function measurements indicated no significant differences with values obtained from prediction equations based on community populations. There were significant across-shift increases in all ventilatory capacity tests (forced vital capacity, 1-second forced expiratory volume, and maximum expiratory flow rates at 50% and the last 25% of the vital capacity) varying from + 1.9% to + 9.8% of the preshift values. Workers older than 40 years of age and those employed for more than 10 years had similar across-shift changes compared with younger workers and those with longer durations of employment. Our data indicate that workers in clean work environments (with the exception of smokers) complain of few acute or chronic respiratory symptoms and have normal lung function. These workers experience increases in lung function across the work shift, probably reflecting normal diurnal variation. Smokers, although significantly more affected than nonsmokers, do not appear to have work-related findings, and smoking appears to be the most significant risk factor for the development of lung disease in unexposed workers. In this study, community-based control data was equivalent to worker control data in unexposed workers.