AIM:To investigate the pathogenesis of aplastic anemia ( AA) through the expression of P 53 and P21, and the telomere length in the bone marrow mononuclear cells (BMMNC) of patients with AA.METHODS: The BMMNC were collected from 60 cases of AA, including 38 non-severe aplastic anemia (NSAA) and 22 severe aplastic ane-mia (SAA), and 25 healthy controls.The mRNA expression of P53 and P21, and the telomere length were detected by real-time quantitative polymerase chain reaction (RT-qPCR), while the protein expression of P53 and P21 was determined by Western blot.The correlations among P53, P21 and telomere length were analyzed .Hematopoietic components of bone marrow were measured by bone marrow biopsy .CD34 +cells proportion in nuclear cells were assessed by flow cytometry . RESULTS:Telomere length in the patients with AA , including NSAA and SAA , was significantly lower than that in con-trol group (P<0.05).The same results of the hematopoietic components of bone marrow and the CD 34+cells proportion in nuclear cells were observed .Telomere length in SAA group was shorter than that in NSAA group ( P<0.05 ) , while the hematopoietic components of bone marrow and the CD 34 +cells proportion in nuclear cells also showed the same results . The expression of P53 and P21 at both mRNA and protein levels in the AA patients , including NSAA and SAA , was signi-ficantly higher than that in control group (P<0.05).The expression of P53 and P21 in the SAA patients was higher signi-ficantly than that in the NSAA patients ( P<0.05 ) .No correlation between the expression of P 53 or P21 and telomere length was found . There was significant positive correlation between the expression of P 53 and P21 ( P <0.05 ) . CONCLUSION:Telomere length, P53 and P21 may be involved in the pathogenesis of AA by inhibiting the proliferation and differentiation of hematopoietic stem cells , and triggering cell apoptosis .