Asthma morbidity is known to exhibit seasonal periodicity. Yet the relationship between trends in morbidity vs mortality is not known. This report describes and compares the seasonal variation in asthma hospitalizations and mortality in the US population. During 1982 through 1986, both mortality and hospitalizations demonstrated periodic seasonal trends that were age specific and did not differ by sex, race, or region. For persons aged 5 through 34 years, hospitalizations peaked in September through November, whereas mortality trends peaked in June through August. A disproportionate number of the summer deaths in this age group occurred out of the hospital. For individuals 65 years old or older, both asthma hospitalizations and mortality demonstrated increases during December through February. The results of this study demonstrate age-specific seasonal variations in the clinical expression of asthma. Perhaps the treatment and prevention of asthma hospitalizations and mortality might be improved through further understanding of these trends.