Delirium is a common syndrome among hospitalized elderly patients. In humans, sleep and circadian rhythms are disturbed during delirium, and both are influenced by the hormone melatonin. This prompted us to investigate the relationship of the clinical subtypes of delirium: 1). hyperactive, 2). hypoactive, and 3). mixed, with melatonin, as reflected by a patient's urinary metabolite 6-SMT. Results support our hypothesis that urinary 6-SMT during delirium was higher in hypoactive and lower in hyperactive patients. Because this is the first time a biochemical parameter related to the clinical subtypes of delirium has been reported, further research on the link between melatonin and delirium is necessary.