Chronic myeloproliferative diseases (CMPDs) associated with t(5;12)(q33;p13) are reported to be responsive to imatinib mesylate. We studied 5 cases of CMPD with isolated t(5;12) treated at our hospital between January 1993 and October 2004. All were men with a median age of 55 years (range, 18-68 years). In the peripheral blood, each had marked leukocytosis, with variable eosinophilia (n = 4), monocytosis (n = 3), or basophilia (n = 2). Bone marrow specimens were hypercellular (70%-100%) with marked myeloid hyperplasia in each patient, but only 1 patient had eosinophilia, monocytosis, and basophilia. Follow-up ranged from 23 to 182 months (median, 48 months). Four died 23 to 182 months after initial diagnosis, 3 of blast crisis and 1 of cardiac complications of severe eosinophilia. Additional cytogenetic aberrations were identified at the time of blast crisis. Of 3 patients treated with imatinib, 2 responded, but only 1 had a sustained response. CMPD with t(5;12) commonly transforms to blast phase, and transformation is associated with cytogenetic evidence of clonal evolution.