The onset of cardiovascular events presents a circadian variation that may be mediated by similar temporal patterns of vascular function. Blood pressure also follows circadian variation. We investigated the possible diurnal variation of endothelial function and arterial stiffness in patients with hypertension. Thirty-five individuals with recently diagnosed hypertension (mean age 48.3 years, range 30-60 years, 14 men) were examined. Flow-mediated vasodilatation (FMD), nitrate-mediated vasodilatation (NMD) and carotid-femoral (cf) pulse wave velocity (PWV) were measured at three different occasions: at 0700 hours immediately after awaking, at 1200 hours and at 2100 hours. FMD was markedly lower in the morning (0700 hours, 2.22+/-1.58%; 1200 hours, 4.37+/-2.25%; 2100 hours, 4.28+/-2.12%; P<0.001), whereas NMD was similar at the same time points. This difference remained significant after adjustment for baseline brachial artery diameter and reactive hyperaemia. PWVcf progressively increased from morning to evening (0700 hours, 9.8+/-1.9 m s(-1); 1200 hours, 10.2+/-2.2 m s(-1); 0900 hours, 10.5+/-1.9 m s(-1); P=0.013 for linear trend). Similar temporal patterns were observed in systolic and diastolic blood pressures peaking in the evening. PWVcf changes lost significance after adjustment for changes in mean blood pressure. Endothelial function is decreased in the early morning in hypertensive patients, whereas arterial stiffness is increased in the evening. Changes in BP-dependent passive artery distension may be involved in this phenomenon.