BACKGROUND: Mutation in the p53 gene occurs frequently in a wide spectrum of human malignancies and the mutant protein may prove to be a useful diagnostic or prognostic marker for acute leukemia. The aim of this study was evaluation of the clinical significance of plasma p53 levels in acute leukemia. METHODS: Plasma p53 protein were measured by an enzyme linked immunosorbent assay in 136 acute leukemic patients and the results were compared with immunohistochemistry on the paraffin-embedded section in 78 cases. In 28 leukemia cases showing increased plasma p53 protein levels at diagnosis, follow-up plasma p53 levels were analysed and compared with morphologic findings of the bone marrow at the same time. RESULTS: (1) In immunohistochemistry, 22 of 78 cases (28%) revealed positive results for p53 protein. (2) Initially positive plasma p53 protein levels were detected in 40 out of 136 (29.4%). There was a moderate agreement (kappa factors 0.3-0.4) between the immunohistochemistry and ELISA results. All 13 cases in which initial positive p53 results were converted to negative at follow-up analysis revealed complete remission. Positive plasma p53 protein levels were converted to negative before the follow-up bone marrow examination in all six cases (acute myeoid leukemia 4 cases, acute lymphocytic leukemia 2 cases) which were analysed serially. (3) The poor prognostic factors reveal no statistically significant difference between the positive and negative plasma p53 groups. CONCLUSION: The results suggest that analysis of plasma p53 level in acute leukemia is a useful test to predict complete remission in addition to the bone marrow examination. Especially, we may expect that all the acute leukemia cases showing positive plasma p53 level at diagnosis and converted to negative level during chemotherapy will enter complete remission.