The prevalence of atopy among patients having chronic fatigue syndrome (CFS) has been reported to be as high as 80% in published surveys of patients with this syndrome. However, many of the reports relied on self-assessment by patients for the presence of atopy or solely used total immunoglobulin E (IgE) levels to assess the likelihood of atopy. To more critically assess the presence of atopy among patients with CFS, testing was done for total IgE and allergen-specific IgE using the Pharmacia CAP system including 20 common allergens: trees (birch/oak/ash), grass (rye/blue), weeds (common/giant ragweed), molds (Penicillium/Aspergillus/Alternaria), dust mites (Dermatophagoides pteronyssinus/Dermatophagoides farinae), animal dander (cat/dog), and foods (egg white/milk/wheat/corn/peanut/shrimp). Testing of 50 patients having documented CFS indicated that 78% had total IgE < 100 IU/mL, among whom 26% had a positive test for allergen-specific IgE of class I or greater for one or more allergens. Among the 22% of CFS patients having a total IgE > 100 IU/mL, 73% had a positive test for allergen-specific IgE for one or more allergens. The most commonly positive allergens were dust mites (24-26%), whereas molds (0-6%) and foods (0-4%) were rarely positive. The overall frequency of positive results for the presence of allergen-specific IgE among CFS patients was 36%, not significantly different from the normal prevalence of these antibodies in the general population (20-35%). This assessment of the prevalence of allergen-specific IgE antibodies in patients with CFS fails to support a potential association between CFS and atopy.