A recurrent specific JAK2 V617F mutation has been reported in bcr/abl-negative chronic myeloproliferative diseases (cMPD), including polycythemia vera (PV), essential thrombocythemia (ET) and idiopathic myelofibrosis (IMF). The mutation is detectable in a variable proportion of neoplastic clones, depending on the molecular methods employed. In this study, we attempted to establish the JAK2 V617F mutation frequency in two partially overlapping cMPD patient series by two different PCR-based techniques. Using an allele-specific polymerase chain reaction assay (AS-PCR), the JAK2 V617F mutation was detected in 124/158 (78.5%) cMPD patients; in particular, 90.2, 72.1, 63.2 and 50% of PV, ET, IMF and unclassified (U)-MPD cases, respectively, showed the mutation. Employing a semiquantitative 5' fluorogenic TaqMan assay, the JAK2 V617F mutation was identified in a much larger percentage of cMPDs patients (118/127: 92.9%). Rates of mutation in PV, ET, IMF and U-MPD cases were 95.9, 85.7, 91.7 and 92.9%, respectively. Furthermore, a statistically higher percentage of JAK2 mutated alleles was detected by TaqMan assay in PV (68+/-3.5, mean value+/-SEM) and IMF (64+/-9.3) cases as compared with ET (35+/-5.4). Finally, a significant correlation between JAK2 V617F mutational status and hematocrit (Ht), white blood cell and platelet counts in PV patients, and Ht values in ET cases, was observed by AS-PCR. Overall, these data indicate that TaqMan technology significantly improved sensitivity in detecting the JAK2 mutation in cMPD patients and may be worth of further evaluations as a clinically useful tool for detection of small amounts of mutated clones.