Chemokines belong to a large family of molecules that are implicated in the localization and production of blood cells. Some of them, such as Interleukin 8 (IL-8) and GRO-a, participate in the regulation of megakaryopoiesis, mostly by exerting an inhibitory action. Recently, the involvement of stromal derived factor 1 (SDF-1) in synergizing the stimulatory effect of thrombopoietin on megakaryopoiesis and its participation in megakaryocyte transendothelial migration has been described. The aim of the present study was the evaluation of the plasma levels of IL-8, GRO- a and SDF-1 in patients with essential thrombocythemia (ET), a myeloproliferative disorder characterized by megakaryocytic hyperproliferation with increased circulating platelet count. Besides, the corresponding chemokine receptors were assayed on platelet membrane from ET patients. A cohort of 27 patients diagnosed according to the Polycitemia Vera Study Group were enrolled in the study (mean age, 45; 21 women). Twenty-seven normal subjects matched by sex and age were taken as the control group. The Ethic Committee from IDIM A. Lanari approved the study and all patients and normal controls signed the informed consent. Plasma levels of the chemokines were measured by ELISA technique (R&D Systems) according to the manufacturer. Expression levels of IL-8 receptors (CXCR1 and CXCR2), GRO-a receptors (CXCR2) and SDF-1 receptors (CXCR4) on platelet membrane were evaluated by flow cytometry using specific MoAbs and the corresponding isotype controls (B-D Pharmingen). Flow cytometry results were expressed as relative fluorescence intensity (RFI, the relationship between the mean fluorescence intensity from the specific antibody and the isotype control). Results were expressed as median and range. Statistic analysis was carried out using Mann-Whitney Wilcoxon rank sum test and Wilcoxon signed rank test. Plasma levels of the chemokines measured in 19 ET patients were similar to that found in normal controls, IL-8 2.5, pg/ml (0.8–28.2) and 2.8 pg/ml (1.1–16.5), GRO-a, 30.0 pg/ml (7.4–463.1) and 23.9 pg/ml (9.6–148.0), SDF-1, 1895.0 pg/ml (1246.0–2719.0) and 1915.0 pg/ml (822–2424.0), respectively. The expression levels of CXCR4 receptor was found diminished in platelets from ET patients, RIF 16.94 (1.3–31.3) compared to normal controls, 27.4 (2.4–58.4); p=0.0059, n=10. However, the expression of CXCR1 and CXCR2 in platelets from ET patients was normal. In conclusion, although plasma levels of the chemoquines IL-8, GRO-a and SDF-1 were normal in these patients, the decreased level of CXCR4 on platelet membrane suggests a dysregulation in the SDF-1a/CXCR4 axis in patients with essential thrombocythemia.