A thorough investigation of the prevalence and incidence of chronic fatigue syndrome (CFS) was conducted by the Centers for Disease Control and Prevention (CDC) in four geographically distinct metropolitan areas in the USA. The humoral immunity of patients with CFS was comprehensively studied in a CDC investigation of an alleged community outbreak. Muscle fatigue and a substantial decrease in exercise tolerance are ubiquitous among patients with CFS. Ineffective treatment modalities for CFS studied in a doubleblind, full-dose placebo-controlled trials in the USA include the antiviral agent acyclovir, a naturopathic regimen of liver extract, folic acid and cyancobalamin, intravenous gammaglobulin, and the antihistamine terfenadine. The term CFS describes a set of nonspecific symptoms associated with persistent or relapsing fatigue. Research studies have confirmed that the majority of patients with CFS are white middle-aged women with a high prevalence of current major depression and somatization disorder and abnormal personality traits.