A high number of circulating CD34+ cells has been advocated to distinguish primary myelofibrosis from other Philadelphia-negative myeloproliferative neoplasms. We reevaluated the diagnostic interest of measuring circulating CD34+ cells in 26 healthy volunteers and 256 consecutive patients at diagnosis for whom a myeloproliferative neoplasm was suspected. The ROC curve analysis showed that a number of CD34+ < 10/mu l excludes the diagnosis of primary myelofibrosis with a sensitivity of 97% and a specificity of 90%(area under the curve: 0.93 [0.89-0.98]; p < 0.001). Patients with PMF harboring a CALR mutation had more circulating CD34+ cells than patients with either a JAK 2 or MPL mutation (p = 0.02 and p < 0.01, respectively). These results suggest that this fast, simple, non-invasive, and standardized test is of particular interest to exclude the diagnosis of primary myelofibrosis.