Comparative genomic hybridization (CGH), using oligo arrays with either 44,000 or 105,000 oligonucleotides, was performed on granulocyte-derived DNA from 71 patients with BCR-ABL-negative classic myeloproliferative neoplasms (MPNs): 32 primary myelofibrosis (PMF), 26 polycythemia vera (PV) and 13 essential thrombocythemia (ET). Copy number changes (CNCs) were detected in 44%, 35%, and 15% of the cases with PMF, PV and ET, respectively. In ET and PMF, CNCs were more frequently detected in the presence of JAK2V617F (50% vs. 19%; p=0.05). Conventional chromosome analysis was obtained in 57 patients either at diagnosis or within 1 year of the array CGH study; all 21 patients with PV and 11 with ET displayed normal cytogenetic findings despite the presence of CNCs in 29% and 18%, respectively. In PMF, the respective rates of CNCs and abnormal karyotype were 48% and 36%; karyotypic abnormalities, including unbalanced translocations, were often detected by array CGH as chromosomal gains or losses. This preliminary report suggests a potential value for array CGH in terms of both clinical diagnostics and genomic research in MPNs.