The complex of the symptoms of psychic disorders and of the disorders of sleep, appetite, and food intake often forms the basis of the clinical picture of a mental disease. However, it is only rarely conceived in a complex manner as a set of physiologically interdependent functions. A remarkable proof of the interdependence of these functions is their complex disorder, the Kleine-Levin syndrome. The first descriptions of the symptoms of the Kleine-Levin syndrome can be found in the studies of several authors published as early as at the turn of the century. In 1942, the syndrome was designated by Critchley and Hoffmann after Willi Kleine and Max Levin, who defined it precisely in 1925 and 1929. The syndrome of periodic hypersomnia, megaphagia, and psychic disorders, originally described only in young males, was later found in females as well; the original very strict criteria were gradually broadened and complemented to some extent. At present, the most commonly accepted criterion for the diagnosis of the Kleine-Levin syndrome is the existence of the combined sleep disorder (hypersomnia or insomnia lasting from days to weeks), food intake disorders (megaphagia or anorexia), and various psychic abnormalities accompanying or following the attacks of the affection. We term the syndrome typical if the sleep disorder appears in the form of hypersomnia, food disorder in the form of megaphagia, and if psychic abnormalities are clearly expressed. On the other hand, we term the syndrome atypical if one of the main symptoms is opposite. The incomplete syndrome consists of only two main symptoms. The attacks of the affection set on mostly suddenly, lasting from several days to several weeks, ending suddenly again. The interparoxysmal periods last from several days to several months, sometimes even to several years. The etiopathogenesis of the affection is still unknown. A number of reports indicate a disorder of the diencephalon, perhaps only of the hypothalamus. The pathological-anatomical findings following the death of persons suffering from the disorders of sleep and food intake and from psychic abnormalities mostly reveal lesions in the region of the third brain ventricle. The development of the typical syndrome is benign, however, and morphological studies are not available. The typical Kleine-Levin syndrome can hardly escape the attention of clinicians owing to the richness and clarity of symptoms. The atypical or discretely expressed forms, however, often remain unrecognized even after a detailed medical examination and may lead to diagnostic uncertainty.