Lymphocyte analysis has not been accepted as an indicator for the clinical diagnosis of paroxysmal nocturnal hemoglobinuria (PNH), because of diverse CD59 expression, even in the healthy individuals. We studied the expression of CD59 on lymphocytes and lymphocyte subsets in healthy individuals and PNH patients by flow cytometry. In 50 healthy individuals, granulocytes generally showed a single-cell population that was strongly positive for CD59. Lymphocytes showed, in addition to the population of positive cells, a small population of weakly positive cells, while CD3(+) T-cells showed a large population of weakly positive cells. Natural killer (NK)-cells showed a profile similar to CD3(+) T-cells. CD4(+) T-cells showed a larger population of strongly positive cells, as compared with CD3(+) T-cells, while CD8(+) T-cells showed a smaller population of strongly positive cells. CD20(+) B-cells showed a similar profile to granulocytes. CD59 expression studies in all our PNH patients showed that B-cells expressed CD59 at levels similar to granulocytes, which were lower than those of T-cells and NK-cells. Therefore, the lymphocyte population with low level CD59 expression in healthy controls corresponds to T-cells (especially CD8(+) T-cells) and NK-cells. CD59 expression on B-cells could thus be used as a new marker for the diagnosis of PNH.