F-18- FDG whole- body positron emulsion tomography (F-18-FDG PET) has become an established imaging modality for a variety of cancers. Today, F-18- FDG PET is utilized to differentiate benign from malignant non-functioning adrenal masses. In this report, a 25-year-old woman presented with a 2-month history of left flank pain. Abdominal CT revealed a left 6.5 x 4.5 cm adrenal mass with regular margins and focal calcification. To make a differential diagnosis, F-18- FDG PET was performed. Preoperative laboratory studies showed that the mass was non-functioning. A left adrenalectomy was performed through a left subcostal incision. The final pathologic evaluation revealed adrenal oncocytoma. We present the disassociation between preoperative F-18- FDG PET and pathologic findings of a benign adrenocortical oncocytoma. Copyright (c) 2008 S. Karger AG, Basel.