Recently, Scinto et al. reported that Alzheimer's disease (AD) patients exhibited a marked hypersensitivity in their pupil dilatation response to a cholinergic antagonist, tropicamide. Recent studies have also revealed the striking association between the apolipoprotein E 4 allele (ApoE4 allele) and sporadic and familial late-onset AD. These observations lead us to examine the association between the pupillary hypersensitivity to tropicamide and ApoE4 allele. Written informed consent was obtained from all normal subjects and AD patients. 23 Japanese patients with probable AD (mean age: 70 years) and 44 healthy Japanese volunteers (mean age: 56 years) were studied here. The clinical diagnosis of probable AD was made by NINCDS-ADRDA criteria. The pupil assay was performed using Iriscorder C2514 (Hamamatsu Photonics, Japan). Briefly, all measurements were performed in subjects who had been dark-adapted for at least 10 minutes. After obtaining baseline data, we administered one drop of 0.01% tropicamide to each right eye and one drop of physiological saline in the other. For each right eye, we recorded pupil areas after 10, 30 and 50 min. DNA was extracted from each subject's peripheral leukocytes for ApoE genotyping by PCR-RFLP. 44 normal subjects were genotyped as: 23 (n=6),33 (n=22), 34 (n=14) and 44 (n=2). 23 AD patients were genotyped as:33 (n=7), 34 (n=12), 44 (n=4). Among normal subjects, the mean % changes in pupil size 50 min after administration of dilute tropicamide increased across ApoE genotypes in this order: 23 (7.7%), 33 (12.2%), 34 (23.6%) and44 (36.2%). Among AD patients, the mean % change in pupil area 50 min after administration of tropicamide was significantly increased (P<0.05) proportional to the number of ApoE4 allele from no ApoE4 allele (20.0%) to a 34 genotype (23.2%) and to a 44 genotype (43.1%). In this study, among both of normal subjects and AD patients, individuals with the ApoE4 allele demonstrated significantly higher increase in pupillary responses to tropicamide than individuals without the ApoE4 allele. These results suggest that ApoE is involved in the mechanism of hypersensitive pupil dilation.