Chronic fatigue syndrome is a recently defined entity for which clinical criteria were proposed by the Centers for Disease Control, Atlanta, Ga. A frequently advocated treatment in Southern California is an injectable solution of bovine liver extract containing folic acid and cyanocobalamin (LEFAC). We conducted a double-blind, placebo-controlled, crossover trial of intramuscular LEFAC in 15 patients who met the Centers for Disease Control criteria for chronic fatigue syndrome. Although patients responded to placebo and LEFAC by several criteria of functional status, no significant difference was apparent between response to placebo and that to LEFAC. The placebo response appeared to be strong.