Objectives: Quantitative regional cerebral perfusion (rCBF) measurements using [¹⁵O]H₂O PET with arterial cannulation and acetazolamide (ACZ) challenge have been reserved to identify high-risk patients that are candidates for by-pass operation. We aimed to assess the prognostic value of various parameters in quantitative [¹⁵O]H₂O PET measurements in patients not subsequently undergoing surgery. Methods: We identified 32 eligible patients who underwent [¹⁵O]H₂O brain PET imaging for suspicion of hemodynamic insufficiency between 2009 and 2020. Cerebrovascular events were defined as new ischemic lesions on MRI, stroke, transient ischemic attack, vascular dementia. Follow-up period was 91 months (range: 26-146). rCBF before (rCBF_base) and after (rCBF_acz) ACZ challenge and the relative increase (CVR), were examined in the anterior (ACA), middle (MCA), and posterior (PCA) cerebral artery territories of the affected hemisphere, and the most recent MRI scans were scored for infarcts and white matter lesions. Results: Receiver operating characteristic (ROC) curve analysis showed higher prognostic accuracy for rCBF_acz(AUC:0.82) compared to CVR (AUC:0.72) and rCBF_base (AUC:0.77). ROC AUC, optimal thresholds (and corresponding sensitivity/specificity/accuracy) for rCBF_acz after ACZ in individual territories were 0.79 and 37.8 mL 100g⁻¹ min⁻¹ (0.81/0.63/0.72) for the ACA, 0.84 and 32 mL 100g⁻¹ min⁻¹ (0.81/0.75/0.78) for the MCA, and 0.70 and 43.9 ml/(mL 100g⁻¹ min⁻¹ (0.81/0.43/0,62) for the PCA. Kaplan Meier survival curve showed longer event-free survival in patients with rCBF_acz below cut-off (p=0.007). In multivariate analysis rCBF_acz remained a significant predictor when correcting for age. Conclusion: Quantitative rCBF measurements after ACZ challenge with [¹⁵O]H₂O PET provided high prognostic value for future cerebrovascular events.