Because of the striking similarity of the clinical manifestations produced by use of the drug reserpine and seen in patients with the chronic fatigue syndrome (CFS), we theorized that CFS was a disorder of reduced central sympathetic drive. Because of the pharmacology of control of this central sympathetic system, we further postulated that CFS symptoms would respond quickly to low dose treatment with a monamine oxidase inhibitor. To test these hypotheses, we designed a randomized, double blind placebo controlled study using phenelzine. No patient in the trial had a diagnosis of lifetime or current psychiatric disorder and none had depressed mood in the range of clinically depressed patients on a paper and pencil test of depression. Patients in the placebo group received placebo for 6 weeks while those in the drug treatment group were treated in three 2-week segments-placebo, 15 mg phenelzine every other day, and then 15 mg daily. This treatment regimen produced a significant pattern of improvement compared to worsening in 20 self report vehicles of CFS symptoms, illness severity, mood or functional status. Thus the data support our hypothesis of reduced sympathetic drive, although an alternative hypothesis of pain alleviation is also possible. The study design also allowed us to evaluate patients for a placebo effect: no evidence for this was found, suggesting that CFS is not an illness due to patients' being overly suggestible.