The chronic fatigue syndrome is a poorly understood condition of uncertain etiology. Researchers disagree on its case definition, possible causes, associated immunologic and psychiatric factors, and treatment. Recent publications suggest possible links with a virus resembling human T-cell lymphotropic virus type 2 and with natural killer cell dysfunction and other cell-mediated immunologic abnormalities. Links with Epstein-Barr virus, human herpesvirus-6, coxsackie B virus, and systemic Candida infections are questioned. Placebo-controlled studies suggest that essential fatty acid therapy may be helpful, disagree on a role for intravenous immunoglobulin, and dismiss oral nystatin and intramuscular bovine liver extracts as therapeutic modalities.