Adult asthma has been the subject of relatively few epidemiologic studies; separation from chronic obstructive pulmonary disease (COPD) has been difficult. Utilizing a cohort of 14,404 subjects, 25 to 74 yr of age, from the First National Health and Nutrition Survey (NHANESI, 1971-75) traced by the NHANESI Epidemiologic Followup Survey (1982-84), we investigated prevalence, incidence, predictors, and consequences of adult asthma. Cases were based on subject reports of active doctor-diagnosed asthma (without COPD). Differentiation of asthma from COPD was partially successful, as suggested by correlations with smoking status and pulmonary symptoms, but was imperfect, as suggested by subsequent hospital experience. Followup interview underestimated interim hospital diagnosis of asthma by 28%. Prevalence of active asthma at NHANESI among U.S. adults was estimated at 2.6%, and followup incidence of new-onset asthma at 2.1/1,000/yr. Low income was the strongest independent predictor of asthma; the higher rates seen in blacks were largely explained by their lower income. Males and females had equal prevalence rates, but females had higher incidence rates. Asthma prevalence and incidence were independent of age and cigarette smoking. Asthmatics were often hospitalized with various lung conditions in the followup period, but unlike those with COPD, did not have a significantly increased risk of death.