A 65-year-old woman was referred to our hospital for fever and diagnosed with pyelonephritis. Abdominal computed tomography showed a right adrenal tumor incidentally, that was 6.5 cm in diameter. We could not rule out malignant disease by magnetic resonance imaging examination and performed resection of the right adrenal tumor. The histopathological examination revealed an adrenal hemangiomatous endothelial cyst, and there was no evidence of malignancy. It was difficult to differentiate between adrenal cyst and adrenal cancer in preoperative diagnostic imaging because the tumor contained hemorrhage and necrotic tissue.