One hundred and forty four patients with a clinical indication of suspected polycythemia vera (PV), essential thrombocythemia, or idiopathic myelofibrosis were screened for JAK2(V617F) and the mutation frequency was 47, 51, and 50%, respectively. Previous investigations enabled 42 of 66 patients with suspected PV to be definitively diagnosed either as PV according to WHO criteria or to have this diagnosis excluded. Ninety-six percent of those with PV were JAK2(V617F), whereas all patients without PV did not have the mutation. Early screening of suspected PV patients for JAK2(V617F) rapidly identifies nearly all those with PV without invasive or less specific conventional investigations.