We report a case of idiopathic CD4+ T-lymphocytopenia with malignant lymphoma (diffuse large, B-cell type) for which there was no evidence of human immunodeficiency virus type 1 or type 2 infection and no other known causes of immunodeficiency. She had never suffered from any opportunistic infection until the diagnosis of malignant lymphoma was made, and the CD4+ T-lymphocytopenia persisted after complete remission of the lymphoma. As the clinical features and immune status of the patient differed from those associated with the acquired immunodeficiency syndrome (AIDS)-related syndrome, we conclude that immunodeficiency in this case did not contribute to the opportunistic infection but may have been associated with the genesis of malignant lymphoma.