Paroxysmal nocturnal hemoglobinuria (PNH) involves the proliferation of an abnormal and possibly premalignant hematopoietic stem cell. Successful treatment of PNH by marrow grafting requires that the PNH clone be eradicated by the pretransplant conditioning regimen. Four patients with PNH-associated marrow aplasia were transplanted with marrow from their HLA-matched, MLR-nonreactive siblings. Three patients were conditioned with cyclophosphamide, procarbazine, and antithymocyte serum (CTX/PCZ/ATS), and one was conditioned with busulfan/CTX/PCZ/ATS. Persistent complete engraftment of myeloid, lymphoid, and erythroid cell lines was demonstrated in all four patients by DNA sequence polymorphism analysis or cytogenetics, and RBC typing. There was no recurrence of the abnormal clone of cells for up to five years after transplantation despite the use of a conditioning regimen in three of them, which is not usually associated with permanent marrow aplasia. Bone marrow transplantation is a curative therapy in patients whose illness is severe enough to warrant the risk.