When a renal lesion is the sole manifestation of systemic lupus erythematosus (SLE), differentiation from other nephropathies is difficult. The membranous form of lupus nephritis is especially difficult to distinguish from idiopathic membranous nephropathy, particularly when multisystem and serologic features of SLE are absent. We report two cases in which the initial renal biopsy findings suggested idiopathic membranous nephropathy and in which the subsequent emergence of SLE might have been predicted by the presence of tubular reticular structures. We identified these structures in 177 of 183 (96.7%) renal biopsy specimens from patients with SLE, but in only three of 128 (2.3%) renal specimens from patients with membranous nephropathy. Tubular reticular structures are markers of the renal lesion of SLE and may be helpful in differentiation from membranous nephropathy.