The levels of immunoglobulin subclasses were determined for 46 patients meeting the original Centers for Disease Control case definition of chronic fatigue syndrome and were compared to values obtained for 50 age- and gender-matched healthy volunteer blood donor controls. The levels of immunoglobulin subclasses in these groups were further compared to a third group of additional chronic fatigue syndrome cases from whom samples had been obtained and frozen prospectively over a period of 7 years. These data do not demonstrate significant immunoglobulin subclass deficiencies in patients with chronic fatigue syndrome.