Background: 18F-FDG positron emission tomography (PET) along with 82Rubidium radiotracer imaging is a non-invasive semi-quantitative technique that aids in the diagnosis of cardiac sarcoidosis. More recently, quantitative analysis using standardized uptake values (SUV) has shown that higher SUV indicates greater degree of active inflammation. This study evaluated the prognostic value of positive PET results on clinical outcome and assessed SUV in relation to PET interpretation and its prognostic value. Methods: We retrospectively analyzed 83 patients with proven or suspected cardiac sarcoidosis, referred for PET imaging between 2009-2013. Baseline characteristics and follow-up events were obtained by medical chart review. Outcomes included death, SCD, heart transplant, drop in EF (>10%), new VT, and ICD discharge. Automated software was used to obtain pixel-based counts from which SUV was calculated. Results: During follow-up, composite endpoints occurred in 23 patients (28%). In multivariate analysis, semi-quantitative assessment of abnormal myocardial FDG uptake was a significant predictor of adverse clinical outcome (HR=3.01, 95% CI=1.06-8.59). This association remained significant in subgroup analysis of Japanese Ministry of Health and Welfare Criteria (JMHC) negative patients (HR=6.96, 95% CI=1.18-40.9). In quantitative assessment, SUV heterogeneity as expressed by the absolute difference in maximum and minimum myocardial SUV value of >0.56 most accurately represented abnormal myocardial FDG uptake (ROC area under the curve 0.82). This was also an independent significant predictor of adverse outcome (HR=3.77, 95% CI=1.15-12.4). Conclusions: Patients with abnormal FDG uptake are 3 times more likely to have adverse outcomes. Abnormal FDG uptake remains a very strong predictor of outcomes in those with negative JMHC. A difference between maximal and minimal SUV of >0.56 g/ml most accurately represents abnormal FDG uptake by semi-quantitative assessment and also predicts adverse outcome. Both semi-quantitative and quantitative assessment of myocardial FDG uptake can be important to clinicians to identify those patients at highest risk for adverse events.