We performed long-term follow-up studies of 5 patients with paroxysmal nocturnal hemoglobinuria (PNH). The percentages of decay-accelerating-factor (DAF)-defïcient erythrocytes were almost stable or slowly increased during the 2-4 years’ observation periods when the effects of treatment with drugs or blood transfusion and pregnancy were excluded. However, a tendency of increase in the percentages of DAF-deficient erythrocytes was observed along with the duration of the disease. Our findings suggest that the growth advantage of PNH clones beyond the normal hematopoiesis may be different from the other clonal stem cell disorders.