Objective:To explore the level of tryptase-positive mast cells in the bone marrow of MDS patients. Methods:Bone marrow biopsies from 10 healthy controls,8 IDA,20 myelodysplastic syndrome and 17 acute myeloid leukemias were embedded in cold plastic resin. Immunohistochemical method was used to show the number of tryptase-positive cells.The level of tryptase-positive cells were compared. Results:No difference was found between the level of tryptase-positive mast cells in IDA group(63.43 ± 44.29) and healthy group(61.43 ± 56.19)(P > 0.05);the level in MDS group(195.95 ± 117.39) was higher than that in control group(P < 0.05), high risk group(229.25 ± 139.68) was higher than low risk group(171.73 ± 98.07),but the difference was not significant(P > 0.05). AML group(608.44 ± 342.58) was higher than that of MDS group(P < 0.01). Conclusion:The number of tryptase-positive mast cells in the bone marrow of MDS patients was increased and may play a role in the pathogenesis and development of the disease.