The EVI1 gene encodes a nuclear, zinc finger, DNA binding protein expressed after provirus insertion into the Fim-3 or EVI1 loci in murine leukemia myeloid cells. EVI1 is also expressed by cells from AML patients with chromosome rearrangements involving band 3q26, the putative location of the EVI1 gene, but expression of this gene is not detected in normal bone marrow. We report expression of EVI1 by cells from a patient with chronic myelogenous leukemia in blast crisis (CML/BC) whose cells showed inv(3)(q22q26). In vitro culture of these cells resulted in macrophage differentiation and loss of EVI1 expression. Results in this patient suggest EVI1 expression played a role in CML blast transformation. Patients with CML/BC and other nonrandom chromosome abnormalities involving chromosome 3q26 should be evaluated for EVI1 expression.