To study the expression and significance of pten gene in patients with myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML), RT-PCR and Western blot were respectively applied to detect pten mRNA and PTEN protein in Jurkat cells (as negative control), in bone marrow nucleated cells of 35 patients with MDS, 45 patients with AML and 20 normal control. The results showed that pten mRNA expression could not be detected in Jurkat cells, and the positive rate in MDS patients (77.1%) was significantly lower than that in normal control group (90.0%) (p > 0.05), while significant difference was found between AML patients and normal control (60.0% vs 90.0%, p < 0.05); the positive rate in MDS-RAEB patients (70.0%) was lower than that in MDS-RCMD (86.7%); positive rate in de novo and relapsed AML patients (53.3%) was lower than that in AML patients in CR (73.3%), but statistics tests did not show significant difference (p > 0.05). The results of relative expression level of pten mRNA in all groups indicated that both relative expression levels in MDS patients and AML patients were definitely lower than that in normal control group (p < 0.005); the relative expression level in MDS-RAEB patients was lower than that in MDS-RCMD patients (p < 0.05); and in de novo and relapsed AML patients was obviously lower than that in AML patients in CR (p < 0.001). However, there was no significant difference between MDS and AML patients (p > 0.05). The positive rate of PTEN protein expression in both MDS (65.7%) and AML (54.8%) patients were lower than that in normal control (90.0%) (p < 0.05), and there was no significant difference when comparing MDS-RCMD patients (80.0%) with MDS-RAEB patients (55.0%) (p > 0.05), but positive rate of PTEN protein expression in de novo and relapsed AML patients (44.4%) was significantly lower than that in AML patients in CR (73.3%) (p < 0.05). It is concluded that the complete loss of pten mRNA in MDS and AML is uncommon, but the relative expression level in both diseases is significantly lower than that in normal people. The positive rates of PTEN protein expression in both MDS and AML patients are lower, compared with normal people, but are not in accordance with the expression of pten mRNA. The abnormalities of pten gene expression may be involved in the pathogenesis of MDS and AML.