Controversial views on the postviral fatigue syndrome ("myalgic encephalomyelitis") were critically appraised in their historical context and recent advances in research (virology, immunology, neurophysiology, histopathology, and epidemiology) reviewed. Flaws detected in certain aspects of recent research included in particular failure to define fatigue, inadequate assessment of psychological features, and absent or inappropriate control groups. The findings suggest that the fruitless dichotomy of "organic versus functional" should be replaced by a multifactorial approach. Most important, epidemiological studies with explicit operational case definition are essential before progress can be made in the management of this distressing disorder.