Non-invasive 24-h blood pressure and a 24-h electrocardiogram were recorded in 45 normotensive and 97 matched, untreated, hypertensive subjects, with and without echocardiographic signs of left-ventricular hypertrophy and without signs of coronary artery disease. Forearm vascular resistance was calculated from mean blood pressure and postischemic blood flow, which was measured by venous plethysmography. Systolic ambulant 24-h blood pressure exhibited the closest correlation with left-ventricular mass index in hypertensives (r = 0.48; p less than 0.001). No relation could be found between blood-pressure fall overnight and left-ventricular hypertrophy. Blood-pressure variability was significantly higher in hypertensive subjects. Furthermore, systolic 24-h blood pressure significantly correlated with the rate of ventricular arrhythmias in hypertensives. Casual blood pressure exhibited no comparable correlations. A significant close correlation between 24-h blood pressure and vascular resistance was identified in hypertensives. Furthermore, left-ventricular mass index and vascular resistance were correlated (in hypertensives: r = 0.32; p less than 0.01). This finding speaks for a parallel development of total peripheral resistance and left-ventricular hypertrophy in essential hypertension. The close relationships between ambulatory 24-h blood pressure, left-ventricular hypertrophy, ventricular arrhythmias, and vascular resistance demonstrates the high diagnostic and, possibly, the prognostic value of long-term blood-pressure measurement.