To determine the relations of 24-hour blood pressure (BP) and its different phases with left ventricular (LV) diastolic filling, 125 subjects (mean age 46 years) not taking cardiac drugs were studied by Doppler echocardiography and ambulatory BP recording. Subjects (excluding those with coronary artery or valvular heart disease, heart failure, or diabetes) were classified into 2 groups according to the level of Doppler-derived ratio of peak early to atrial velocity (E/A ratio): 59 had E/A >1 (normal diastole), 62 had E/A <1 (impaired diastole), and 4 had E/A = 1. Patients with E/A <1 were older and had higher LV mass indexed for height, average 24-hour BP, average nighttime BP, and lower day-night BP decrease, whereas average daytime BP did not differ significantly between the 2 groups. Negative correlations of E/A were found with age, heart rate, office, average 24-hour and average nighttime systolic and diastolic BP, and LV mass index. In a multivariate model that included potentially confounding factors, only age (standardized beta coefficient = -0.52, p<0.00001), nighttime BP (beta = -0.28, p<0.0001), and heart rate (beta = -0.22, p<0.001) were independent predictors of E/A in the pooled population. In conclusion, LV diastolic function is more closely related to ambulatory, rather than to clinic, BP measurements, and high average nocturnal diastolic BP is a powerful marker of LV filling impairment.