OBJECTIVE To determine whether potential alteration in p53 function through p53 gene mutation or mdm-2 overexpression correlates with early treatment failure in childhood acute lymphoblastic leukemia (ALL). METHODS Diagnostic marrow samples from 34 children were analyzed for p53 gene alterations by western blot and SSCP/DNA sequence analysis and for mdm-2 overexpression by western blot analysis. These samples were derived from two groups of children with ALL: 17 good outcome patients who are in long-term continuous complete remission and 17 poor outcome patients who did not achieve a complete remission or relapsed within 6 months of achieving remission. RESULTS Two children within the poor outcome group were found to have p53 gene mutations. Furthermore, five poor outcome patients were shown to have greater than 10-fold overexpression of mdm-2 protein compared with the mean level of mdm-2 protein measured in the good outcome group. Aberrant p53 protein expression was found in only one good outcome patient, whereas no good outcome children were found to have elevated levels (> 10-fold) of mdm-2 protein. CONCLUSIONS We show for the first time that potential alteration in p53 function in childhood ALL is more common (P = .036) in cases of early treatment failure than in children who remain in long-term continuous remission.