The clinical and genetic relationships between Alzheimer's disease (AD) and glaucoma remain obscure. The aim of this study was to determine the prevalence of open-angle glaucoma (OAG) in patients with AD and whether the apolipoprotein E (APOE) 4 allele is associated with AD, with or without OAG, in Japanese. The groups consisted of 172 patients with the diagnostic criteria of AD and 176 age-matched controls. Ophthalmic examinations were conducted, and genomic analysis was performed by PCR and digestion of products with an enzyme. OAG was found in 41 (23.8%) of the AD patients, which was a significantly (p = 0.0002) higher prevalence than that in the controls (9.9%). Furthermore, there was no significant difference between intraocular pressures (IOPs) in AD patients with OAG and without OAG. The percentage of AD patients who carried an APOE epsilon4 allele (29.5%) was significantly (p = 0.0007) higher than that of the controls (9.1%). However, the percentage of AD patients with OAG who carried an APOE epsilon4 allele (35.7%) was not significantly different than that of AD patients without OAG (27.7%, p = 0.42). In summary, the prevalence of OAG is high in Japanese patients with AD, suggesting that common factors other than APOE may contribute to the two diseases.