Asthma is the second most common chronic respiratory disorder seen by physicians in outpatients, and in 1988 was estimated to be the primary or secondary cause of 9.3 percent of all physician office visits for ambulatory medical care in the United States. Reports citing increased asthma morbidity and mortality in this country and abroad have intensified interest in this disorder. Investigations into possible explanations for these increases, however, have been hampered by the lack of good longitudinal data on the incidence, prevalence, and health care utilization associated with asthma, and by the lack of generally accepted diagnostic criteria for defining the disease.