Long-term clinical remission of more than 10 years is rarely seen in paroxysmal nocturnal hemoglobinuria (PNH). Affected blood cells in PNH lack glycosylphosphatidylinositol (GPI)-anchored membrane proteins such as decay-accelerating factor (DAF) and CD59. We performed a flow cytometric analysis of circulating blood cells obtained from two patients with PNH who had been in clinical remission for more than 10 and 25 years, respectively. Affected cells with the PNH phenotype were demonstrated only among T-lymphocytes. Persistent affected T cells were negative for the CD52 protein only, this protein being a GPI-anchored lymphocyte marker without complement regulatory activity. The persistence of the affected T cells may be explained either by an inherently long life span after the disappearance of the PNH stem cell or by insidious production at a subclinical level by affected stem cell. In either event, detection of affected T cells, especially CD52-negative T cells, may be useful for the evaluation of long-term clinical remission in PNH.