A report from England has suggested that Chronic Fatigue Syndrome exists in equines and constitutes an emerging veterinary problem. Preliminary epidemiological studies seem to confirm the zoonotic implications of CFS. An arsenical drug, sodium thiacetarsamide, was administered to four horses with a diagnosis of Chronic Fatigue Syndrome (CFS), already treated unsuccessfully with different medications. The CFS-like lethargy, with accompanying symptoms and signs, of the four animals obtained a complete remission after intravenous treatment with this drug at low dosage (0.1 mg/kg/day). No adverse side effects were ever noticed. This clinical response was associated with recovery from anaemia and decrease of muscular enzyme values in two of the four horses. In all patients, micrococci-like bacteria found before treatment adhering to the outer surface of many red blood cells, disappeared at post-treatment controls. Considerations are made on the possible action of an arsenical drug, used in isolation, in the treatment of CFS.