A 64-year-old Japanese male patient with lupus nephritis associated with adult T cell leukemia (ATL) is described. Percutaneous renal biopsy demonstrated findings consistent with membranous lupus nephritis. To our knowledge, this is the first case of lupus nephritis complicated by ATL, suggesting that human T cell leukemia virus type I may be correlated not only to outbreaks of ATL but also to lupus nephritis/systemic lupus erythematosus.