Acute lymphoblastic leukemia (ALL) is thought to arise from the clonal expansion of a single transformed precursor cell. However, an oligoclonal Ig heavy chain (IgH) rearrangement pattern has been observed in 30% of ALL patients and was shown to be the result of ongoing rearrangement events. The extent and nature of these ongoing rearrangement processes in individual patients has so far remained obscure. We performed a detailed analysis of leukemic VHDJH rearrangements in three children with B-precursor ALL at diagnosis and one B-lymphoid blast crisis of a child with Ph+ chronic myeloid leukemia at diagnosis and relapse. The children were selected because they presented with multiple IgH rearrangements on Southern blot analysis. Polymerase chain reaction analysis of leukemic cells from two B-precursor ALL patients showed exclusively two groups of related sequences resulting from VH gene replacement events. Most VH gene replacements involved 3' located acceptor VH genes. Analysis of cells from the other B-precursor ALL patient showed exclusively related sequences as a result of VH gene joinings to a pre-existing DJH rearrangement. In the B-lymphoid blast crisis, a single germline precursor cell had generated multiple unrelated rearrangements and additional groups of related rearrangements resulting from VH to DJH joinings. Direct proof for the VH to DJH joining mechanism was obtained by amplification of the expected preexisting DJH rearrangements. Our findings suggest that the pattern of ongoing rearrangements in an individual patient reflects the IgH rearrangement status of the precursor cell at the time of malignant transformation. Sequence analysis of VHDJH rearrangements at diagnosis may therefore allow a prediction of the reliability of complementarity determining region 3 probes for the detection of minimal residual disease.