The chronic fatigue syndrome is a heterogeneous disorder characterized by easy fatigability, feverishness, diffuse pains, and depression. Many patients also report inhalant, food, or drug allergies. This article reviews the clinical features of the syndrome and hypotheses of its pathogenesis, especially those regarding the Epstein-Barr virus and cellular immune mechanisms. Also summarized are recent studies of the validity of atopic complaints in the syndrome. The results of epicutaneous skin testing demonstrated a high correlation with history in 24 patients. Atopy coexists with the chronic fatigue syndrome in greater than 50% of patients.