Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired hemolytic anemia resulting from a somatic mutation in a hemopoietic stem cell. In most cases of hemolytic PNH, the majority of the marrow cells are derived from the PNH clone. Recent evidence has indicated, however, that the majority of the most primitive peripheral blood stem cells (PBSCs) in PNH appear to be of normal phenotype. This has led to tentative suggestions that normal PBSCs could be collected and used for autologous transplantation. We have investigated this possibility in four PNH patients by treating them with granulocyte colony-stimulating factor (G-CSF) in an attempt to mobilize normal progenitors. The expression of glycosylphosphatidylinositol (GPI)-linked proteins was analyzed by flow cytometry on mature neutrophils, late stem cells (CD34+/CD38+), and primitive stem cells (CD34+/CD38-). The phenotyping and stem cell quantitation was performed in steady-state blood and post-G-CSF administration. The most primitive PBSCs (CD34+/CD38-) were almost all normal before G-CSF treatment, even when the patients' neutrophils were mainly PNH. However, after G-CSF, the cells that were mobilized into the peripheral blood were of a similar phenotype to the mature neutrophils, ie, mainly PNH. It is possible that PNH-stem cells are preferentially destroyed by complement in the peripheral blood leaving only normal cells in the circulation. After G-CSF, the PNH cells in the marrow are released into the blood. Our findings suggest that it would be difficult to collect sufficient numbers of normal stem cells for autologous transplantation.