We report a case of adrenal oncoctyic pheochromocytoma in a 37-year-old woman. The patient presented with a 2-year history of an increase in abdominal girth. Computed tomographic studies revealed a large left adrenal mass, which was subsequently excised. Grossly, the tumor measured 17 x 14 x 8.5 cm, weighed 1,150 g, and had a solid, brown cut surface. Histologically, it consisted of large polygonal tumor cells containing eosinophilic granular cytoplasm and arranged in nesting, alveolar, and trabecular patterns. Electron microscopy revealed closely packed mitochondria and dense-core membrane-bound granules in almost all tumor cells. The latter were immunohistochemically positive for chromogranin, synaptophysin, neuron-specific enolase, neurofilament, serotonin, bombesin, ACTH, vimentin, desmin, S-100 protein, and cytokeratins, including AE1/3, CAM 5.2, cytokeratin 7, and cytokeratin 20. To the best of our knowledge, this is the first reported case of adrenal oncocytic pheochromocytoma confirmed by ultrastructural study. The immunoreactivity of this tumor adds several unusual features to the wide immunohistochemical spectrum of pheochromocytoma.