Serum concentrations of immunoglobulin E (IgE) were measured in 31 children with bronchial asthma and in 55 healthy children. Among the asthmatic children, 70% had elevated IgE levels, 83% had a low titre of histamine-latex agglutination and 80% had blood eosinophilia over 3%. All asthmatic children showed hypersensitivity to one or more allergens; the highest incidence of hypersensitivity was to house dust (38% of asthmatic children), followed by pollens (25%) and vegetable fibers (19%). No relation was observed between serum IgE concentration and the intensity of cutaneous reaction or the number of sensitizing allergens. No significant difference was observed in serum IgE concentration in asthmatic children submitted hyposensitization therapy compared with those who had not received such treatment. It is concluded that IgE determination may be valuable for differential diagnosis between allergic and non allergic asthma and that IgE measurement should be considered as a complementary test in the investigation of allergy.