The peak incidence of cardiovascular complications in the morning points to a possible role of the abrupt increase in blood pressure on arising. However, there is as yet no firm evidence linking the extent of the elevation in blood pressure on arising and the risk of cardiovascular complications. We sought a correlation between blood pressure on arising and left ventricular mass in a population of 181 previously untreated hypertensive patients. Ambulatory blood pressure was measured over a 24-h period, and each patient was requested to trigger a blood pressure determination immediately after standing on arising in the morning. Left ventricular mass was measured with M-mode echocardiography and indexed for height, height(2.7), and body surface area. The systolic blood pressure on arising was significantly (P <.01) better correlated than office blood pressure with left ventricular mass index and wall thickness. On multivariate analysis, the values of systolic blood pressure on arising and mean 24-h systolic blood pressure contributed significantly and independently to the correlation with left ventricular mass and wall thickness. These observations point to the significance of the arising blood pressure. A marked abrupt daily elevation in blood pressure on arising, then maintained for a certain time, could contribute to the development of left ventricular hypertrophy and may constitute a trigger for cardiovascular complications.