In children under six years of age referred to a pediatric allergy practice the usefulness of serum IgE assay was evaluated in relation to age, symptoms, nasal eosinophilia, skin tests and family history. In more than 60% of the children the initial symptom, usually rhinorrhea, had occurred before one year of age. In infancy the diagnosis was more difficult, gastrointestinal complaints were more frequent and nasal eosinophilia less frequent than in the older children. Many infants had positive skin tests to foods and to environmental allergens. There was a significant correlation between elevated serum IgE level and age, nasaeosinophilia, the number of positive skin tests and the probability of immunotherapy being prescribed. Although no clear diagnostic level is seen, an IgE level above 100 micron/ml at any age and an IgE level above 20 micron/ml in infants strongly suggest the possibility of atopic disease. However, a low IgE level does not exclude atopic disease.