To explore the natural history of asthma and its relation to allergic responses, we examined the relation between total serum IgE in early adulthood and a history of respiratory symptoms, airway hyperresponsiveness (AHR), and atopy during childhood. We studied 180 subjects aged 18-20 years who had been studied since the age of 8-10 years. We measured wheeze in the previous year by questionnaire, AHR by histamine inhalation test, atopy by skin prick tests, and serum IgE levels by immunoassay. Subjects with AHR in early adulthood had higher IgE levels (mean 257.0 IU/ml) than subjects with past AHR (mean 93.3 IU/ml) or with lifelong normal responsiveness (mean 67.6 IU/ml) (P < 0.001). Subjects who had symptoms had higher IgE levels (mean 125.9 IU/ml) than those who were lifelong asymptomatic (mean 63.1 IU/ml) (P < 0.001). Recent wheeze, AHR, and allergic sensitization all had a positive relation to serum IgE, but IgE was not more predictive of AHR than skin prick tests. The finding that young adults who are sensitized to common allergens are highly likely to have AHR even in the absence of symptoms is further evidence of the fundamental role of IgE-mediated responses in the natural history of AHR throughout childhood and into adulthood.