We report herein a case of idiopathic adrenal hematoma. A 59-year-old Japanese man was referred to our hospital for evaluation of a 7.0 cm mass in the right upper abdominal cavity. The tumor was suspected to originate from either the posterior segment of the liver or the right adrenal gland. His chief complaint was weight loss of 8 kg over the previous 6 months. He had no past medical history and took no medications, including no anticoagulants. Laboratory data were almost normal except for a slight elevation of PIVKA-II. The origin of the tumor was found to be the adrenal gland, as angiography revealed the blood supply to the mass to derive from the right superior and inferior adrenal arteries. A fine needle biopsy of the lesion was unable to confirm the diagnosis. Open right adrenalectomy was performed. The histopathological findings of the surgical specimen revealed a hematoma with normal adrenal tissue. In the absence of any obvious etiology, the diagnosis was idiopathic adrenal hematoma.