Circulating endothelial cells (CECs) have been studied in cardiovascular disorders and as a marker of angiogenetic activity; clinical applications are limited by a lack of consensus on their phenotypic identification and quantification. We determined CECs in essential thrombocythemia (ET) patients, to investigate their possible pathogenetic role. We considered CECs as CD146⁺/CD45⁻ nucleated cells, detected in peripheral blood from 21 healthy controls and 39 ET patients, performing a combination of pre-enrichment of CD146⁺ circulating cells and multiparametric flow cytometry measurement (FCM). Levels of CECs in ET patients were higher with respect to controls (median 2844 CECs/mL vs. 121.3 CECs/mL, P < 0.0001). Apparently hydroxyurea treatment did not influence the levels of CECs. As another established marker of endothelial activation, we also assessed soluble E-selectin (sE-selectin) levels in 31 of the ET patients and compared with 39 healthy volunteers: median sE-selectin level in ET patients was 35.3 ng/mL, higher with respect to controls (24.48 ng/mL), P = 0.0369. Our data suggest that endothelium in ET is activated, reflecting a significant role of angiogenesis in this disorder and suggesting an important endothelial contribution in the hypercoagulable state of ET patients.