OBJECTIVE To analyze the percentage and functional changes of natural killer (NK) cell subsets in peripheral blood of severe aplastic anemia (SAA) patients before and after immunosuppressive therapy (IST) so as to evaluate the relationships between these changes and hematopoietic functions and explore the role of NK cells in the pathogenesis of SAA. METHODS By flow cytometry, the percentages of NK cells (CD3(-)CD56(+)CD16(+)) and its subsets [CD3(-)CD56(bright)CD16(-)(CD56(bright)), CD3(-)CD56(dim) CD16(+)(CD56(dim)), CD3(-)CD56(-)CD16(+)] in peripheral blood lymphocytes were detected in 12 untreated patients, 30 recovered patients and 13 normal controls respectively from April 2010 to December 2010 in our hospital. NK cells activating receptors (NKG2D and NKp46), perforin and granzyme-β of patients and normal controls were also detected. The correlation between these changes and hematopoietic functions, including the percentages of neutrophil granulocyte (ANC%), lymphocyte and reticulocyte absolute value in peripheral blood, and hyperplasia degree, percentage of granulocytes, erythrocytes, lymphocytes and megakaryocytes absolute value in bone marrow were evaluated. RESULTS (1) The percentages of NK cells (10.30% ± 6.08%) and CD56 bright cells (0.11%) in untreated patients were significantly lower than those of recovered patients (16.47% ± 8.29%, 0.68%, both P < 0.05) or normal controls (19.45% ± 6.88%, 0.53%, both P < 0.05). The percentage of CD56(dim) cells in untreated patients was significantly lower than that of normal controls (9.62% ± 6.04% vs 18.21% ± 7.16%, P < 0.05). The percentage of CD3(-)CD56(-)CD16(+) cells was significantly higher in recovered patients than that of untreated patients or normal controls (0.79% vs 0.37%, 0.41%, both P < 0.05). (2) The expression of NKp46 and perforin of NK cells in untreated (88.23%, 64.97% ± 21.61%) and recovered patients (82.97%, 66.14% ± 20.73%) were significantly higher than those of healthy controls (40.99%, 42.11% ± 27.25%, all P < 0.05). (3) The percentage of NK CD56(bright) and CD3(-)CD56(-)CD16(+) cells of patients was positively correlated with ANC% (r = 0.423, 0.609, 0.468 respectively, all P < 0.05) and the percentage of granulocytes in bone marrow (r = 0.357, 0.517, 0.434 respectively, all P < 0.05). The percentages of NK, CD56(bright), CD56(dim) and CD3(-)CD56(-)CD16(+) cells were positively correlated with the hyperplasic degree of bone marrow (r = 0.455, 0.412, 0.404, 0.451 respectively, all P < 0.05), but they were negatively correlated with the percentage of lymphocytes in bone marrow (r = -0.522, -0.435, -0.411, -0.547 respectively, all P < 0.05). The expression of NKG2D, NKp46, perforin and granzyme-β of NK cells had no correlation with hematopoiesis (all P > 0.05). CONCLUSIONS The lowered percentage of NK CD56(bright), CD56(dim) cells and a higher expression of perforin may cause the over-function of T lymphocytes and thus lead to hematopoietic failure in SAA.