Chronic fatigue syndrome and nickel allergy

50 patients with chronic fatigue syndrome (CFS) and 73 controls were patch tested with 8 metal allergens. We found an overrepresentation of allergies among the CFS patients, which was not significant. However, allergy to nick el occurred in 36% of patients in the CFS group and in 19% of subjects in t he control group (p<0.05). The high frequency of nickel allergy was more no teworthy in females in the CFS group than among female controls (52% and 24 %, respectively, p<0.05). Similarly, in the males the figures were 14% and 9%. We suggest that in vivo immunoactivation by ions of nickel, or metal cr oss-reacting with nickel, could be an etiological factor in CFS.