Chromosomal aberrations in polycythemia vera (PV) are heterogenous and nonrandom. A prognostic predictive value of these aberrations has not been established. The V617F mutation in the JAK2 gene on chromosome 9p24.1 was identified recently in peripheral blood leukocytes in the majority of patients with PV and in approximately half of patients with essential thrombocythemia and idiopathic myelofibrosis. Within the JAK2 V617F-positive PV patients, however, clinical presentation and degree of myeloproliferation varies to a great extent. Here we report four cases of chronic myeloproliferative disorders [two with PV, one with PV in transformation to idiopathic myelofibrosis (IMF) and one IMF patient], with the distinct karyotypic abberations der(18) t(9;18) (p13;p11) and der(9;18)(p10;q10). Two patients had hyperproliferative PV and two had "transitional PV" and IMF, respectively. All four patients harbored the JAK2 V617F mutation. Our data, together with previously published data, clearly indicate an association of these chromosomal abnormalities with a highly proliferative PV phenotype with a propensity to transform into postpolycythemic myelofibrosis. Cytogenetic analysis seems to identify a subgroup of patients with a distinct prognostic profile, and should be performed in conjunction with a JAK2 mutation analysis in patients suspected of a chronic myeloproliferative disease.