Patients with polycythaemia vera (PV) are at increased risk of developing arterial and venous thromboembolic complications. We investigated whether endothelium-dependent, flow-mediated vasodilatation (FMD) is impaired in PV patients without clinical evidence of artery disease as observed in patients with conventional cardiovascular risk factors. FMD and endothelium-independent, nitroglycerine-induced vasodilatation (NMD) were assessed using high-resolution ultrasound in the brachial artery of 20 patients with PV and 20 sex- and age-matched control subjects (CTL). FMD was markedly impaired in PV patients compared with CTL (7·6 ± 2·9% versus 11·6 ± 5·7%, P = 0·009) whereas NMD was similar in both study groups. The impairment of FMD was independently related to the presence of PV (r = −0·434, P = 0·009) and vessel size (r = −0·107, P = 0·038) but was not related to haematocrit values and platelet counts. The results demonstrate that PV is associated with endothelial dysfunction in the pre-clinical phase of arterial disease. However, the precise mechanisms by which PV leads to this altered vascular reactivity remain unclear.