Abstract： Objective To learn more about the clinical and laboratory features of patients with paroxysmal nocturnal hemoglobinuria (PNH) diagnosed in the past ten years. Methods Clinical and laboratory data for 78 cases of PNH diagnosed from January 1990 to November 1999 in our hospital were analyzed retrospectively. Results In comparison with PNH cases reported in the 1980s, the newly diagnosed PNH cases showed the following features: (1) older age of disease onset (from 27 to 34 years); more female cases (from 18.5% to 38.5%); more cases without hemoglobinuria (from 24.2% to 38.5%). (2) No positive family hereditary history. (3) Bone marrow dysplasia, abnormal karyotype and negative sister chromatid differentiation were found in 19.2%, 12.2% and 8.9% of the PNH patients, respectively. 12.3% of the patients had bone marrow hypoplasia, and most of them had no hemoglobinuria. Ham's tests were negative in about 34.2% of the cases. CD55 and CD59 on peripheral blood cells were deficient in 100.0% of the cases, suggesting that CD55 and CD59 tests can improve the diagnosis of PNH. (4) Adrenocortical hormone was effective in 83.8% of the patients, 54.2% of whom relapsed within one year. Eight refractory and relapsed patients were treated with low dose chemotherapy (MP therapy: Melphalan 2-6?mg*d-1; Prednisone 0.5?mg*kg-1*d-1). Five (62.5%) of them showed positive responses. Bone marrow failure and other side effects were not serious in this group of patients. Conclusions PNH, an acquired blood disease seen more often among adult males, can be diagnosed more sensitively by hemocyte member CD55 and CD59 tests and treated more effectively with adrenocortical hormone or low dose chemotherapy.