The development of an effective new case definition for the chronic fatigue syndrome poses a unique challenge because of the protean nature of fatigue and its accompanying symptoms. Unfortunately, the 1988 case definition failed to identify a unique disease entity (Hickie and colleagues; Reyes and colleagues. Unpublished data). Although the case criteria suggested by Fukuda and colleagues [1] address many of the shortcomings of the 1988 research definition, the clinical and social need for a definitive diagnostic tool remains largely unmet.

Although the previous research case definition was a significant step forward, its clinical implications were unanticipated. Many physicians were left wondering whether their patients had the syndrome or not. Many patients were denied benefits because they did not "make the diagnosis."

The difficulty in finding commonality among fatigued patients was highlighted in the deliberations of the International Chronic Fatigue Study Group. Differences of opinion yielded to the common sense of the need to describe a range of illness. Although the goal of the process was the development of an improved research definition, the study group was mindful of the clinical ramifications of such a publication. The construct that emerged is an attempt to provide a vehicle for epidemiologic study and clinical research, as well as language to facilitate patient care.

Lest we fail to learn from the shortcomings of the previous case definition, the limitations of the new definition must be kept clearly in mind. It is, first and foremost, a research definition that will nonetheless have an important clinical effect. It is not license to include or exclude patients from any particular treatment or benefit on the basis of a classification such as chronic fatigue syndrome or idiopathic chronic fatigue. The establishment of the subgrouping came from the study group's intuitive sense that patients with the syndrome may prove somehow unique among fatigued patients. Such a finding remains to be seen.