Serum IgE levels were measured by a paper-disc radioimmunoassay technique (PRIST) in 425 nonallergic subjects and in 570 patients with asthma, 244 with allergic rhinitis, 48 with asthma and eczema (atopic dermatitis), 49 with eczema but without asthma, and 57 with chronic urticaria. The data are presented for age groups in geometric means and standard deviations and by modal distribution. Normal mean IgE levels for the total sample were 32 IU/ml with highest levels (mean of 51 IU/ml) in school-age children. The highest IgE levels were found in patients with both asthma and eczema (mean of 985 IU/ml), followed by asthma alone (305 IU/ml), eczema alone (273 IU/ml), and allergic rhinitis (171 IU/ml). The values for our United States population were higher than those reported from Scandinavian countries but lower than those reported from Canada. The geometric mean plus 1 SD (64 IU/ml for infants, 150 IU/ml for schoolchildren, and 100 to 120 IU/ml for all other age groups) appears to be the most useful limit of normalcy. Overlaps with normal values are largest for urticaria, eczema, and allergic rhinitis and least for patients with allergic asthma.