In order to establish reference values for total serum IgE in an adult non-atopic population, 175 individuals, 17-85 years of age, were investigated. The usefulness of a sensitive method (PRIST) for serum IgE determinations in discriminating atopy from non-atopic conditions in allergological routine diagnosis was elucidated by investigating 445 patients with symptoms of asthma, rhinitis, urticaria and eczema. Comparisons were made with case histories, in vivo tests and circulating IgE antibodies. The geometric mean for serum IgE in the reference material was 13.2 kU/l with a 2 SD range of 1.53 to 114 kU/l. No significant difference between age groups or sexes was observed. In patients classified as non-atopic and pronounced atopic, the geometric mean values for IgE were 40, 123 and 458 kU/l respectively. The IgE level correlated with number of allergens positive in RAST and with skin test results. It is concluded that IgE determinations are of great help in discriminating atopic conditions from other diseases with similar symptoms. A serum IgE value above 100 kU/l in a patient is strong evidence for the presence of an atopic disease while a value below 20 kU/l indicates that the symptoms are due to intrinsic or infectious disease.