We have investigated alterations of the p53 gene in human leukemias by polymerase chain reaction-mediated restriction fragment length polymorphism analysis. This method detects the codon 72 polymorphism of the p53 gene, allowing identification of loss of heterozygosity (LOH) of the p53 gene. In this study, at least two specimens were obtained from each patient to compare the allele status at different points of clinical course. Of 21 cases examined 7 were heterozygous at this polymorphic site and were evaluable for LOH study. Only one patient of Philadelphia chromosome-positive acute lymphoblastic leukemia (ALL) lost the heterozygosity in the specimen of her last relapse. Leukemic cells from her repeated relapses including the last one revealed to have the clonally rearranged immunoglobulin H chain gene of the same size, indicating that alteration of the p53 gene in this patient might account for the lethal relapse as a clonal evolution event. Northern-blot analysis of the p53 gene showed that one case of CD7(+) ALL had altered p53 mRNA. Overall, it is demonstrated that alterations of the p53 gene might be involved in the pathogenesis or progression of at least some human leukemias, although the alterations in leukemias seemed to be not as frequent as in solid tumors.