The etiology of chronic fatigue syndrome (CFS) remains an enigma. But literature concerning chronic fatigue which does not focus on CFS points to all sorts of enzyme deficiencies as possible causes. The deficiencies are probably dismissed as causes of CFS because other characteristic symptoms are lacking in CFS patients. But these symptoms are often also lacking in patients with a deficiency. Symptom patterns in enzyme deficiencies are extremely variable. Therefore, patients with CFS should be screened systematically for enzyme deficiencies. (C) 2000 Harcourt Publishers Ltd.