Chronic fatigue syndrome (CFS) is characterized by unexplained, debilitating fatigue or easy fatigability lasting longer than six months. While a number of clinical trials have been performed in CFS patients, there is currently no established therapy for CFS. Treatment with acyclovir of CFS patients is ineffective. Intravenous immunoglobulin therapy appears to be effective, though the results are controversial. Antidepressants might help the associated depression and anxiety but not other symptoms. Trials with magnesium have improved the well-being of patients. Restoration of NK activity by biological response modifiers, such as sizofirann, resulted in restoration of NK cell activity and recovery from CFS. Taken together, immunological abnormalities may be involved in CFS, and its restoration may produce clinical benefit in CFS.