The aim of this study was to identify risk factors for rupture of unruptured cerebral aneurysms (UCAs) in elderly Japanese patients aged 70 years or older. The participants included all patients 70 years of age or older in 3 prospective studies in Japan (the Unruptured Cerebral Aneurysm Study of Japan UCAS Japan, UCAS II, and the prospective study at the Jikei University School of Medicine). A total of 1,896 patients aged 70 years or older with 2,227 UCAs were investigated. The median and mean follow-up periods were 990 and 802.7 days, respectively. The mean aneurysm size was 6.2 ± 3.9 mm. Sixty-eight patients (3.6%) experienced subarachnoid hemorrhage during the follow-up period. Multivariable analysis per patient revealed that in patients aged 80 years or older (hazard ratio HR, 2.02; 95% confidence interval CI, 1.16-3.49, p = 0.012), aneurysms 7 mm or larger (HR, 3.08; 95% CI, 1.35-7.03, p = 0.007 for 7-9 mm; HR, 7.82; 95% CI, 3.60-16.98, p < 0.001 for 10-24 mm; and HR, 43.31; 95% CI, 12.55-149.42, p < 0.001 for ≥25 mm) and internal carotid-posterior communicating artery aneurysms (HR, 2.45; 95% CI, 1.23-4.88, p = 0.011) were independent predictors for UCA rupture in elderly patients. In our pooled analysis of prospective cohorts in Japan, patient age and aneurysm size and location were significant risk factors for UCA rupture in elderly patients.