Purpose of review: To review the conceptual problems in distinguishing between undifferentiated somatoform disorder and chronic fatigue syndrome, for both may present with fatigue as the main symptom.
Recent findings: The differences and/or similarities between undifferentiated somatoform disorder and chronic fatigue syndrome have not been studied, conceptually or empirically. The literature fails to present discriminant validity of chronic fatigue syndrome in relation to undifferentiated somatoform disorder. A critical feature is implied in the definition of undifferentiated somatoform disorder but absent from the definitions of chronic fatigue syndrome: some patients experience their fatigue as being exclusively physical and not as mental, which is prima facie peculiar, for fatigue is necessarily a mental experience. One is not able to experience fatigue without a mind (or a brain). This experience is characterized as a 'mindless' fatigue, underpinned by pathological reductionist thinking. By not recognizing this critical feature, diagnostic endeavours may perpetuate the problem as a function of the patient's difficulty.
Summary: Proponents of chronic fatigue syndrome should distinguish chronic fatigue syndrome from undifferentiated somatoform disorder, if chronic fatigue syndrome is a distinct entity at all. Further, the 'mindless' quality is a critical feature that needs consideration in refining the concept of undifferentiated somatoform disorder.