['To define an association of bone marrow microvessel density (MVD) with histological properties (the magnitude of fibrosis and quantification of megakaryocytes (MGKC)) in patients with Ph-negative chronic myeloproliferative diseases (CMPD).', 'MVD was analyzed in 93 patients with different forms of CMPD, by estimating histological parameters. True polycythemia (TP) was present in 28 patients; 20 patients had essential thrombocythemia (ET), 36 had subleukemic myelosis, out them 6 were in a prefibrotic stage, and 9 with diagnosed post-TP (ET) myelofibrosis. The grade of myelofibrosis was estimated from the degree of bone marrow fibrosis as 0, 1, 2, and 3 and the clusters of MGKC were in accordance with degrees: 0, 1, and 2. MVD was studied from the absolute number of CD34-positive vascular structures.', 'In patients with TP, fibrosis was defined as grade 0 and 1 in 23 (82%) and 5 (18%) cases, respectively. The content of reticulin fiber was in the normal range in 19 (95%) of the 20 patients with ET. The clusters of MGKC of grades 1 and 2 showed an even distribution among patients with ET and those with TP. Fibrosis was absent in all the patients (n = 6) with prefibrotic-stage primary myelofibrosis (PMF). The patients with PMF had high MVD values [6.5 (range 2.8-22)] than those with TP [4.0 (range 1.76-10.2)] or ET [3.7 (range 2-8.5)] and the controls [3.2 (range 2-4.1)] (p < 0.001) confirming that angiogenesis is uninvolved at the onset of disease in patients with ET and those with TP. The patients with prefibrotic-stage PMF had higher values [6.0 (range 4.8-10.6)] than those with ET [3. 7 (range 2-8.5)] (p < 0.001). This suggests that angiogenesis is an early sign preceding the development of fibrosis.'] Is [ Study of bone marrow microvessel density one of the diagnostic approaches in patients with Ph-negative chronic myeloproliferative diseases ]?