Secondary lymphoma of the breast is a rare entity in patients with non-Hodgkin's lymphoma (NHL). HIV infection is associated with an increased risk for developing NHL, however lymphomatous involvement of the breast in AIDS patients has rarely been reported. We present the case of a 33-year-old HIV-infected female patient with diffuse NHL who presented with a unilateral breast mass. Histologic examination of the biopsy specimen revealed a highly-malignant diffuse large B-cell lymphoma.