Only 17 cases of oncocytic adrenocortical carcinoma have been reported in the English literature. Here, we report an incidental case of oncocytic adrenocortical carcinoma. The patient was a 69-year-old man with the chief complaint of abdominal pain. Abdominal computed tomography revealed a left adrenal tumor. No hormonal symptoms were observed. The excised tumor was whitish, encapsulated, and 75 x 60 x 45 mm in size. Large polygonal tumor cells were arranged in a generally diffuse architecture and exhibited abundant eosinophilic granular cytoplasm. Nuclear atypia with atypical mitotic figures and capsular and sinusoidal invasions were observed. The tumor cells were immunopositive for vimentin, neuron-specific enolase, and synaptophysin but not for alpha-inhibin, melan A, or p53. Diffuse and strong immunopositivity with an antimitochondrial antibody proved that this tumor was truly oncocytic. Upon review of previous cases of oncocytic adrenocortical tumors, we reconsidered the diagnostic findings of the potential for malignancy.