OBJECTIVES: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.