A patient with Ph-negative chronic myeloid leukemia showed active karyotypic evolution when he entered blast crisis. One cell line, which predominated briefly in an accelerated myeloid phase, was characterized by the t(11;19)(q23;p13). Chromosome in situ hybridization demonstrated movement of the oncogene c-ets-1 from the der (11q-) to the der (19p+). The breakpoint at 19p13 was in the vicinity of the human insulin receptor gene locus (INSR). No rearrangements of the c-ets and INSR genes were found in Southern blot analyses. Myeloid lineage was indicated by cell morphology and absence of immunoglobulin JH gene rearrangement and was supported by loss of the germ line bcr-3' gene. Chromosome rearrangements involving 11q23 and movement of c-ets-1 characterize monocytic and lymphoid leukemias and have not previously been reported in myeloid blast crisis of chronic myeloid leukemia.