Introduction: Philadelphia-negative myeloproliferative neoplasms (Ph-MPN) are characterized by overproduction of one or more blood cell lines. Methods: We studied the proliferative characteristics of 91 patients with de novo Ph-MPN. Colony-forming cells (CFC) and endogenous colonies (EC), from bone marrow (BM) and/or peripheral blood (PB), were analyzed by colony assay based on methylcellulose. The level of circulating CD34(+) cells was determined by flow cytometry. Results: The total number of PB CFC in primary myelofibrosis (PMF) was increased compared to the control sample (P < 0.01) and essential thrombocythemia (ET) (P < 0.05). The highest number of BM and PB EC was observed in polycythemia vera (PV) (P < 0.01). Increased levels of CD34(+) cells characterized early-prefibrotic (57%) and advanced-fibrotic PMF (90%) as compared to PV (34%) and ET (32%) (P < 0.01). In the whole Ph-MPN group, the total number of PB CFC (P < 0.01), PB EC (P < 0.05), and CD34+ cells (P < 0.01) correlated with the degree of BM fibrosis. Higher levels of circulating CD34(+) cells in PMF correlated with the total number of PB EC (P < 0.05) and degree of BM fibrosis (P < 0.01). Conclusions: Exploration of the PB proliferative characteristics of Ph-MPN on diagnosis may be helpful in revealing early-prefibrotic PMF. Monitoring the levels of circulating CD34(+) cells may provide a sensitive indicator of fibrotic evolution in PV and PMF.