The increased tendency to fall asleep during the daytime together with increased wakefulness during the night has been demonstrated in patients with advanced Alzheimer's disease (AD). The aim of this study was to assess daytime sleep propensity in a cohort of patients with mild/moderate AD and to correlate it with cognitive impairment. Twenty drug-free AD patients meeting the NINCDS-ADRDA criteria for probable AD were evaluated. According to their Clinical Dementia Rating scores, subjects were classified into mild (CDR1; n=11) and moderate (CDR2; n=9) dementia patients. A group of 12 healthy subjects was taken as controls. The subjects were evaluated by the multiple sleep latency test (MSLT) after their nocturnal sleep pattern had been assessed by a polysomnographic recording throughout the night before. Both groups of AD patients showed a higher level of daytime sleepiness, which was statistically significant for mean daytime sleep latency (MDSL) (controls versus CDR1 and versus CDR2, CDR1 versus CDR2) and for 10:00 and 12:00 hour naps (controls versus CDR1, controls versus CDR2). In the entire group of AD patients, MDSL was significantly related with MMSE, De Renzi's Token test, verbal fluency, verbal digit span, story recall, Raven's Progressive Matrices, Weigl test and Benton's three-dimensional test. These data indicate that an increased sleep propensity during daytime occurs also in patients with mild/moderate AD detected by objective neurophysiological techniques.