PURPOSE: To reassess the usefulness of 18F-FDG PET in the differential diagnosis of benign and malignant tumors from adrenal masses detected on CT or MR. MATERIALS AND METHODS: A retrospective analysis was performed on 20 patients (n = 21), on whom PET scans were obtained with characteristically benign CT (n = 21) and MR (n = 2) findings. Seventeen patients had a proven primary malignancy and three patients had adrenal incidentalomas. PET findings were interpreted as positive if the 18F-FDG uptake of the adrenal mass was greater than or equal to that of the liver. Each adrenal mass was characterized by its size and standardized uptake value (SUV). For statistical analysis, the t-test was used to analyze results for size and SUV. RESULTS: PET findings were positive for eight adrenal masses; two masses were pathologically proven as adrenocortical oncocytomas. The false positive rate was 38% in all patients and 41% in patients with a malignancy. CONCLUSION: 18F-FDG PET was useful in evaluating the primary lesions as well as metastases, but a high false positive rate in patients with a primary malignancy should be considered in the diagnosis of metastasis to the adrenal gland.