While serum immunoglobulin E (IgE) concentration has been shown to be related to allergic disease, there have been few studies of the natural history of IgE development in young children. This study compared total and allergen-specific serum IgE concentrations, by gender, from birth through 4 years of age using a population-based cohort of normal children followed from 1987 to 1993. Subjects were enrolled from a geographically defined area through membership in a large health maintenance organization, and they resided in the northern suburbs of Detroit, Michigan. Total IgE concentrations were measured at birth and at 2 and 4 years of age. Specific IgE concentrations to selected common inhalant allergens (dust mite (Dematophagoides farinae), cat, ragweed (Ambrosia artemisiifolia), and timothy grass (Pheleum pratense)) were measured at 2 and 4 years of age. Total IgE increased with age from birth to 4 years for boys and girls, and was higher in boys at 2 and 4 years of age. Girls demonstrated significant increases in IgE to mite and cat and decreases in IgE to ragweed and timothy from 2 to 4 years of age, whereas there was little change for boys except an increase in IgE to ragweed. Differences in IgE development between young boys and girls may partially account for the higher prevalence of asthma in boys than in girls.