Delirium is common among hospitalized elderly patients, with a prevalence estimated between 15% and 50%.1,2 This syndrome is characterized by disturbances of consciousness, attention, and global changes in cognition. Moreover, abnormalities of mood, perception, and behavior are frequent. All these symptoms fluctuate during the day. The sleep patterns of delirious patients are usually disturbed as well. At times, the entire sleep—wake cycle is reversed: patients are drowsy and nap during the day, whereas night sleep is short and fragmented.3 Melatonin, a hormone with hypnotic effects that increase the propensity to sleep and its duration and quality, is one factor that influences human sleep patterns.4,5,6,7,8 Melatonin also seems to be related to the biological regulation of the circadian rhythms.9,10,11,12,13 Thus, disturbances of sleep and circadian rhythms, as seen in delirious patients, and the known influence of melatonin on these physiological parameters warrant the search for a possible link between the delirium syndrome and melatonin. To date, the only study investigating a possible link between delirium and melatonin is that of Mukai et al.,14 who studied patients with alcohol withdrawal delirium and reported changes in melatonin production during the delirium. Although these changes were specific for the delirium period, their pattern was inconsistent. According to clinical expression, the delirium syndrome can be classified into three subtypes: hyperactive, hypoactive, and mixed.15 Given the known effects of melatonin, we hypothesized that some relation could exist between melatonin blood levels and the subtypes of delirium. A high level of melatonin may be expected in the hypoactive type and a low level in the hyperactive type. The aim of this study was to measure urinary 6-sulphatoxymelatonin (6-SMT) as an indicator of the melatonin level in all three groups of delirious patients and to look for a link between the melatonin level and the clinical subtypes of this syndrome.