At presentation, bone marrow specimens from over 25% of B-lineage acute lymphoblastic leukemias (ALL) display more than two clonal rearrangements of immunoglobulin heavy chain (IgH) genes in Southern blot analyses. Nucleotide sequence analysis has shown predominantly different V(H)DJ(H) junctions among these genes, leading to the frequent description of such cases as oligoclonal leukemias. In the present study, we have analyzed the lgH genes from four patients whose leukemic cells contained different patterns of lgH gene rearrangements between presentation and relapse. Nucleotide sequence analysis of the lgH genes showed that three mechanisms could account for these differences: de novo V(H)DJ(H) rearrangement, V(H) to DJ(H) recombination, and V(H) replacement. In all cases, more than two totally different V(H)DJ(H) rearrangements appeared during evolution of the disease, formally consistent with the conclusion that these tumors were composed of apparently unrelated clones. However, the retention of some of the antigen receptor gene rearrangements, as well as the persistence of a chromosomal marker in two cases, indicated that these leukemias had a monoclonal origin. These findings support the hypothesis that some ALLs arise from a lymphoid progenitor cell at a stage of lymphocyte development before the onset of IgH gene rearrangement. These leukemic lymphocyte progenitors generate malignant daughter cells capable of an in vivo maturation that involves the completion of multiple different lgH rearrangements as well as the modification of preexisting rearrangements by V(H) to DJ(H) recombination or by a V(H) replacement.