Addison's disease due to adrenal lymphoma usually manifests as bilateral adrenal enlargement. We report a patient with Addsion's disease in whom the initial overt primary adrenal insufficiency was accompanied by an only slightly enlarged right adrenal gland. The 80-year-old man presented with nausea, anorexia, weight loss, and hyperpigmentation of the skin and buccal mucosa. Addison's disease was diagnosed based on this clinical presentation and laboratory findings of low cortisol and high adrenocorticotropin levels. Computerized tomography (CT) of the adrenal glands revealed a small right adrenal tumor. His family refused to allow percutaneous or surgical biopsy to determine the nature of the tumor. His general condition improved after steroid supplementation. However, about 1 year later, dizziness, fever, night sweats, and edema of the lower legs developed, and adrenal CT scanning revealed that the left adrenal gland had enlarged and now exceeded the size of the right one. Left adrenalectomy was performed and pathology showed a diffuse large B-cell lymphoma. Staging work-up using whole-body CT scanning suggested a stage IIIb lymphoma. Chemotherapy was given, but the disease still progressed and the patient died 4 months after diagnosis. Primary adrenal lymphoma should be considered in the differential diagnosis of Addison's disease, even if only slight enlargement of the adrenal glands is found initially.