Cardiovascular problems are a major cause of morbidity and mortality in patients with autosomal dominant polycystic kidney disease (ADPKD). Endothelial dysfunction (ED), which is an early manifestation of vascular injury, has been shown in patients with ADPKD. However, the association between ambulatory blood pressure and ED has not been investigated in these patients. Forty-one patients with ADPKD having well-preserved renal function were included in the study. Ambulatory blood pressure monitoring was performed in all patients. Patients were divided into dipper and non-dipper groups. Endothelial function of the brachial artery was evaluated by using high-resolution vascular ultrasound. Endothelial-dependent dilatation was expressed as the percentage change in the brachial artery diameter from baseline to reactive hyperemia. The mean 24-hour systolic blood pressure was similar in both groups (125.5 +/- 10.7 mmHg in dippers and 121.2 +/- 14.3 in non-dippers, p > 0.05). There was also no significant difference between the mean 24-hour diastolic blood pressures in both groups (82.3 +/- 9.6 mmHg in dippers and 77.1 +/- 8.6 mmHg in non-dippers, p > 0.05). The nocturnal fall rate in systolic blood pressure was 11.1 +/- 1.2% in dippers and 0.98 +/- 0.9% in non-dippers (p = 0.001). The nocturnal fall rate in diastolic blood pressure was 14.0 +/- 0.9% in dippers and 3.8 +/- 0.8% in non-dippers (p = 0.001). Endothelial-dependent dilatation was significantly higher in dippers compared to non-dippers (6.22 +/- 4.14% versus 3.57 +/- 2.52%, p = 0.025). Non-dipper patients with ADPKD show significant ED, which has an important impact on cardiovascular morbidity and mortality.