Chronic fatigue and immune dysfunction syndrome (CFIDS) is a recognized human illness with zoonotic implications that is rarely described in animals. Eight birds of prey examined between 1992 and 1995 and sharing common symptoms (asthenia, inability to fly, poor appetite and emaciation) underwent laboratory tests revealing immunodeficiency, anaemia, high creatine kinase levels and low serum magnesium levels. Diagnosis of CFIDS was based upon these features. The effectiveness of an arsenic-based medication, thiacetarsamide sodium, administered intravenously for 2–3 days at low dosages (0.1 ml/kg/day) has been demonstrated by checks carried out 10, 20 and 30 days after therapy. The symptoms and the immune and haematological dysfunctions disappeared within 2–4 weeks of treatment. In all patients, micrococcus-like organisms found adhering to the outer surface of many red blood cells, had disappeared at post-treatment controls. Two of five blood cultures were positive for Staphylococcus spp. (S. intermedius and S. xilosus). Consideration is given to the pharmacological activity of an arsenic-based drug in animal illnesses resembling CFIDS.