http://www.ncbi.nlm.nih.gov/pubmed/23480562 Abstract Introduction: The World Health Organization has classified Myalgic Encephalomyelitis (ME) as a neurological disease since 1969 considering Chronic Fatigue Syndrome (CFS) as a synonym used interchangeably for ME since 1969. ME and CFS are considered to be neuro-immune disorders, characterized by specific symptom profiles and a neuro-immune pathophysiology. However, there is controversy as to which criteria should be used to classify patients with "Chronic Fatigue Syndrome." Areas covered: The Centers for Disease Control and Prevention (CDC) criteria consider chronic fatigue (CF) to be distinctive for CFS, whereas the International Consensus Criteria (ICC) stresses the presence of post-exertion malaise (PEM) as the hallmark feature of ME. These case definitions have not been subjected to rigorous external validation methods, for example, pattern recognition analyses, instead being based on clinical insights and consensus. Expert opinion: Pattern recognition methods showed the existence of three qualitatively different categories: (a) CF, where CF evident, but not satisfying full CDC syndrome criteria. (b) CFS, satisfying CDC criteria but without PEM. (c) ME, where PEM is evident in CFS. Future research on this "chronic fatigue spectrum" should, therefore, use the abovementioned validated categories and novel tailored algorithms to classify patients into ME, CFS, or CF.