We have examined the prevalence of incidence of asthma and other wheezing syndromes in subjects in a longitudinal epidemiologic study. The point prevalence of asthma was 6.6%, with the highest rates occurring in children. Rates were also relatively high in older subjects, in most of whom "chronic bronchitis and/or emphysema" had been concomitantly diagnosed. Other wheezing was very common in this population sample; in most age groups, the point prevalence rates of some form of wheezing exceeded 30%. New asthma developed in 1.4% of the subjects who were followed over a period of approximately 4 yr. New attacks of shortness of breath with wheeze occurred in 10.3% of the subjects at risk over the same time period. The incidence of asthma was greatest in young children, was least in late adolescence, and increased again in early adult life. The incidence was 1.5 times greater in young boys than in young girls but was much greater in women older than 40 yr of age, perhaps reflecting the diagnostic biases of physicians. In subjects younger than 40 yr of age, onset of the disease was strongly associated with previously demonstrated allergy skin test reactivity. New disease in this age group occurred de novo, primarily within the first few years of life or during early adult life. Subjects in whom asthma developed after 40 yr of age usually had prior symptoms of chronic bronchial irritation and often had obvious spirometric abnormalities. The disease in these subjects was not associated with positive allergy skin test reactions. Because in these older subjects it does not appear possible to clearly distinguish "asthma" from "chronic bronchitis," the label "asthmatic bronchitis" appears to be a reasonable descriptive term for this syndrome.