Recommended "normal limits" for serum IgE generally assume that a single upper limit of normal can be applied to all adults. The present article describes the distribution of IgE levels in 2657 subjects in a general population sample in Tucson, Ariz. Limits of IgE defining the lower 5%, 10%, 25%, 50%, 75%, 90%, and 95% in 1569 subjects with negative allergy skin tests and without current asthma, considered a reference group, are provided by age and sex. These cutoffs are then used to compare groups with asthma and with positive allergy skin tests with the reference population. Distributions of IgE levels in these groups are vastly different, but defining an upper "limit of normal" for serum IgE is of doubtful clinical value because there is no single level of IgE that distinguishes different groups with any precision. The spread of IgE values is extremely wide in subjects with and without known allergic diseases.