The superiority of ambulatory blood pressure monitoring over casual blood pressure measurement for the prediction of target-organ damage is now well established, although the significance of "white coat" hypertension is still controversial. Is an office blood pressure measurement that is higher than the mean ambulatory value an added risk? Because left ventricular hypertrophy is a potent risk factor, the effect of hypertension on left ventricular mass merits attention. Left ventricular mass indexed for height was measured by M-mode echocardiography in 204 essential hypertensive patients (140 males, 64 females; mean age [SD], 50 [11] years). Blood pressure over 24 hours was monitored routinely with a noninvasive auscultatory device. Office blood pressure after 10 minutes of the subject in the supine position was measured by the same physician under the same conditions. The white coat effect was defined as the difference between the office and average daytime values of systolic and diastolic blood pressures. Patients were divided into four quartiles according to their office and daytime blood pressures. Two-way analysis of variance was used to assess the influence of the white coat effect at a given level of daytime blood pressure on left ventricular mass. Left ventricular mass increased significantly (P < .001) with daytime systolic blood pressure but was independent of white coat effect for both the whole population and the 143 untreated patients. We conclude that the white coat effect has no influence on left ventricular mass, favoring the view that white coat hypertension is a benign condition.