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.