To the Editor: In our recent review article on chronic fatigue syndrome, we concluded that it is a complex disorder with a possibly heterogeneous etiology. To date, no single cause for chronic fatigue syndrome, whether a virus or a significant psychological event, has been identified. However, the bulk of scientific studies over the last two decades have identified a host of pathophysiological abnormalities across many domains. Although the role and significance of each of these abnormalities in predisposing to the illness, directly causing the symptoms, or occurring because of the condition are unclear, Dr. Berger’s suggestion that psychological factors are the single cause of chronic fatigue syndrome and other functional somatic syndromes is rather simplistic. In fact, Wessely, a key figure in drafting the Report of the Joint Committee of the Royal Colleges of Physicians, Psychiatrists, and General Practitioners (1997), specified that "chronic fatigue syndrome cannot be considered either ‘physical’ or ‘psychological’" and has continuously argued for a biopsychosocial approach to the illness (1). Additionally, several mechanisms, including the central and neuroendocrine nervous systems, have been identified in functional somatic syndromes in general (2) and fibromyalgia specifically (3). Thus, an understanding of the etiology of functional somatic syndromes and fibromyalgia is far from unanimous, and the issue of psychological factors as the single cause for these syndromes has not been settled, as asserted by Dr. Berger.