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Purpose: Although some parameters of positron emission tomography with 18F-fluorodeoxyglucose (18F-FDG) and computed tomography (PET-CT) are somehow helpful in differentiating malignant pleural effusion (MPE) from benign effusions, no individual parameter offers sufficient evidence for its implementation in the clinical practice. The aim of this study was to establish the diagnostic accuracy of a scoring system based on PET-CT (the PET-CT score) in diagnosing MPE. Methods: One prospective derivation cohort of patients with pleural effusions (84 malignant and 115 benign) was used to develop the PET-CT score for the differential diagnosis of malignant pleural effusion. The PET-CT score was then validated in another independent prospective cohort (n = 74). Results: The PET-CT parameters developed for discriminating MPE included unilateral lung nodules and/or masses with increased 18F-FDG uptake (3 points); extrapulmonary malignancies (3 points); pleural thickening with increased 18F-FDG uptake (2 points); multiple nodules or masses (uni- or bilateral lungs) with increased 18F-FDG uptake (1 point); and increased pleural effusion 18F-FDG uptake (1 point). With a cut-off value of 4 points in the derivation cohort, the area under the curve, sensitivity, specificity, positive likelihood ratio, and negative likelihood ratio of the PET-CT score to diagnose MPE were 0.949 (95% CI: 0.908–0.975), 83.3% (73.6%–90.6%), 92.2% (85.7%–96.4%), 10.7 (5.6–20.1), and 0.2 (0.1–0.3), respectively. Conclusions: A simple-to-use PET-CT score that uses PET-CT parameters was developed and validated. The PET-CT score can help physicians to differentiate MPE from benign pleural effusions. [ABSTRACT FROM AUTHOR]