The relationship between allergy and asthma in infants and preschool children was investigated. One hundred nine children, median age 2 1/2 years, were examined immunologically by quantitation of serum IgE, RAST testing to 13 allergens, and culture of peripheral blood lymphocytes for spontaneous IgE formation. We examined a cross-section of infants and toddlers to determine whether the severity of asthma is associated with allergy as has been reported in older children with asthma. We identified a group of highly atopic infants and preschool children who have serum IgE at least 10 times the mean + 1 SE for age, multiple positive RASTs with early formation of IgE to inhalant antigens, and circulating B cells that spontaneously form IgE when these are cultured in vitro. Such highly atopic infants and preschool children were statistically more likely to have chronic asthma requiring multiple continuous medication compared to the rest of the population (p less than 0.01).