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Purpose: To determine the diagnostic performance of PET-CT in differentiating benign and malignant adrenal lesions when evaluating PET parameters individually as well as in combination with CT parameters, using histopathology as the reference standard., Methods: 18 F-FDG-PET-CT scans of patients undertaken within 6 months prior to pathologic evaluation of their adrenal lesion(s) were evaluated. PET assessments consisted individually of maximum standardized uptake value of the adrenal lesion (A-SUV max ) and its ("normalized") ratio to the liver (R-SUV max ). The diagnostic performances of these two PET parameters were also assessed when combined with the Hounsfield density from the non-contrast CT component of the PET-CT (A-HU). Diagnostic performance was assessed by area under the curve (AUC) of the receiver operating characteristics. Multiple logistic regression analysis was used to evaluate the individual and combined parameters., Results: The study cohort consisted of 61 adrenal lesions (59 patients). Malignant lesions (n = 52) had significantly higher median PET and CT parameters than benign lesions: A-SUV max (11.4 vs. 6.1), R-SUV max (3.3 vs. 1.7), and A-HU (37 vs. 24) [all p < 0.023]. AUC for the PET parameters individually was almost identical: 0.75 for A-SUV max and 0.74 for R-SUV max . On univariate analysis, thresholds of A-SUV max >3.47 and R-SUV max >0.83 yielded maximum accuracy (both 87%). The combination of these PET parameters individually with A-HU improved both AUC and accuracy (0.81% and 93%, respectively)., Conclusions: The individual PET parameters A-SUV max and R-SUV max have similar diagnostic performance for differentiating malignant and benign adrenal lesions; their performance and accuracy improve when combined with the CT component (A-HU).