A 65-year-old woman visited our hospital complaining of general fatigue and nausea. CT scan revealed a homogeneous mass in the left adrenal gland, which was seven centimeters in diameter. Mild swelling of the right adrenal gland was also suspected. We failed to find the primary tumor, although a metastatic non-functioning adrenal tumor was suspected. Adrenalectomy was performed under the diagnosis of a non-functioning adrenal tumor. Pathological examination showed a non-Hodgkin's lymphoma. Since a bleeding tendency gradually developed following the operation, a bone marrow biopsy was done, revealing an invasion by tumor cells. Patients with a malignant lymphoma involving the bone marrow should not be operated on because fatal complications may develop postoperatively. A malignant lymphoma should be considered as a possible diagnosis of adrenal tumors, although it is very rare.