Objective To determine the expression of Cyclin D 1 protein in patients with acute leukemia, and to evaluate the relationship between Cyclin D 1 expression and clinical treatment outcomes. Methods Cyclin D 1 protein was examined by immunohistochemical method in 48 acute leukemia patients, among whom 32 were ANLL and 16 were ALL, male 31, female 17, aging from 12 to 70, years with a mean of 34 4 years. It was also assayed in five normal bone marrow samples. Results There is no Cyclin D 1 protein detected in normal bone marrow cells. The positive rate of this protein in the patients is 31 3%(15/48), 25 0%(8/32)in ANLL and 43 8%(7/16)in ALL, respectively. There was no significant difference between the positive rate of the patients with ANLL and that with ALL. The positive rate of Cyclin D 1 protein was highest in the relapse group and lowest in the CR group, and the expression degree was higher in the relapse group than that in patients at the time of diagnosis. Conclusion Overexpression of Cyclin D 1 does exist in patients with leukemia, and its expression degree is correlated with progression of the disease, especially with relapse. Cyclin D 1 protein may be a prognostic factor relating to poor outcome in our study