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Introduction: This post hoc subanalysis of the randomized Japanese Primary Prevention Project investigated whether once-daily low-dose aspirin versus no aspirin reduced the risk of cardiovascular events (CVEs) in patients aged ≥ 70 years with atherosclerotic risk factors. Methods: Patients aged < 70 years (young-old) or ≥ 70 years (old) with hypertension, dyslipidemia, or diabetes participated between 2005 and 2007. Patients were randomized 1:1 to receive 100 mg enteric-coated aspirin once daily or no aspirin plus standard of care. The primary outcome was a composite of death from cardiovascular causes plus nonfatal stroke and nonfatal myocardial infarction. The secondary outcome was a composite of the primary outcome plus transient ischemic attack, angina pectoris, and arteriosclerotic disease requiring medical or surgical intervention. Old (n = 7971) and young-old (n = 6493) patients were followed up for a median 5.02 years. Results: Aspirin did not reduce the risk of primary (hazard ratio [HR] 0.92 [95% confidence interval {CI} 0.74–1.16]; P = 0.50) or secondary (0.85 [0.70–1.04]; P = 0.11) outcomes in patients aged ≥ 70 years. In old men with high-density lipoprotein < 40 mg/dL, treatment with low-dose aspirin was associated with a reduction in the incidence of the primary endpoint compared with the group not receiving aspirin (10/260 vs 22/250; HR 0.44 [95% CI 0.20–0.93]; P = 0.03). This subgroup was also found to contain significant larger proportions of patients with elevated body mass index, patients with diabetes mellitus, and smokers (P < 0.001). Old patients also showed differences in bleeding outcomes. Serious extracranial hemorrhage requiring transfusion or hospitalization occurred significantly more frequently in the aspirin-treated group than in the non–aspirin-treated group (35 [0.88%] vs 18 [0.45%]; HR 1.96 [1.11–3.46]; P = 0.020). Gastrointestinal hemorrhage occurred significantly more frequently in the aspirin-treated group than the non–aspirin-treated group (63 [1.58%] vs 18 [0.45%]; relative risk [RR] 3.5 [2.08–5.90]; P < 0.0001). Cerebral hemorrhage (intracranial hemorrhage) tended to occur more frequently in the aspirin-treated group than the non–aspirin-treated group (22 [0.55%] vs 11 [0.28%]; RR 2.01 [0.97–4.14]; P = 0.058). Cerebral hemorrhage occurred significantly more frequently in old patients than in young-old patients (33 [0.41%] vs 10 [0.15%]; HR 2.7 [1.34–5.53]; P = 0.0055). Gastrointestinal hemorrhage occurred in a slightly higher proportion of old patients compared with young-old patients (81 [1.02%] vs 53 [0.82%]; RR 1.2 [0.88–1.76]; P = 0.21). Discussion/Conclusions: Aspirin did not reduce the risk of the primary or secondary outcomes in old patients. Aspirin treatment may have reduced CVEs within a high CVE risk elderly population subgroup. Aspirin treatment in such a group requires caution, because of the increased risk of intracranial hemorrhage, severe extracranial hemorrhage requiring hospitalization or transfusion, and gastrointestinal bleeding in old patients receiving aspirin therapy. Clinical Trial Registration: The study is registered at ClinicalTrials.gov [NCT00225849]. [ABSTRACT FROM AUTHOR]