This study was purposed to explore the clinical efficiency and side effects of GHA (G-CSF, homoharringtonine and low-dose cytarabine) priming chemotherapy for patients with refractory acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS), and its relationship with B7.1 expression. 79 cases of refractory AML and 21 cases of MDS were treated with GHA standard priming chemotherapy. Clinical efficiency, side effects, and therapy-relevant mortality were observed in comparison with MA therapy. Expression of costimulatory molecule B7.1 was detected by immunofluorescence and its relationship with clinical efficiency was explored. The results showed that the remission rate in AML was 60.7% (complete remission rate was 43% and partial remission rate was 17.7%), and that was 52.4% in MDS. The incidence of granulocyte deficiency was 25% during 3.5 days. The severe infection rate was 3%, without severe bleeding, with mild digest effect, and slight damage of function in heart, liver, and kidney. The therapy-related mortality was zero. The higher CR rate was in AML-M(2) and AML-M(5) (60.9%, 61.9%), and the longest remission period was 4 years; expression rate of costimulatory molecule B7.1 displayed large variance (0% - 66.7%) and had positive correlation with efficiency of priming chemotherapy. The rate of B7.1 expression was higher in AML-M(2) and AML-M(5) and lower in other AML groups and normal control. It is concluded that GHA priming chemotherapy can be used to treat refractory AML and MDS, without severe side effects, toxicity and therapy-related mortality. It is a new chemotherapy protocol with better effect and low toxicity. Efficiency of GHA priming chemotherapy may be correlated with B7.1 expression. Its mechanism is worthy to be further explored.