Abdominal ultrasound examination in an 11-year-old, intact, female Labrador dog with hepatic disease revealed a nodular swelling of the left adrenal gland. Hyperadrenocorticism was suspected, but endocrine tests were negative. At the owner's request, an adrenalectomy was performed. Grossly, a nodular mass protruded from the external surface of the left adrenal gland and in cut section was hemorrhagic and effaced the cortical and medullary regions. Histologic examination revealed a cortical neoplasm with medullary involvement. The mass was composed of well-differentiated adipose cells, megakaryocytes, hematopoietic cells, and macrophages containing hemosiderin deposits. A diagnosis of cortical adrenal myelolipoma was made.