The present study investigated whether the morning-evening difference in self-measured blood pressure (BP) (MEdif) can be an independent determinant of cardiac damage in untreated hypertensive patients. In a cross-sectional study, the left ventricular (LV) mass, relative wall thickness, and diastolic function using echocardiography were assessed in 356 untreated hypertensive patients. Home BP measurements were taken in triplicate in the morning and evening, respectively, for 14 consecutive days with a memory-equipped device. Thereafter, the association between the MEdif in systolic BP (SBP) and the echocardiographic parameters was assessed. The MEdif in SBP was significantly correlated with LV mass index (r = 0.28, P < 0.001), relative wall thickness (r = 0.21, P < 0.001), ratio of E-wave to A-wave (r = -0.24, P < 0.001), and the deceleration time of the E-wave velocity (r = 0.23, P < 0.001). In a multivariable regression analysis, the MEdif in SBP was a significant determinant of these parameters, independent of age, sex, duration of hypertension, current smoking, habitual drinking, diabetes mellitus, the average of morning and evening SBP, and the heart rate at echo. When the MEdif in SBP was divided into quartiles, the highest quartile had increased likelihood of LV concentric hypertrophy (odds ratio = 2.63, 95% confidence interval = 1.20-5.87, P = 0.008) in comparison with the lowest quartile after adjusting for confounding factors. The MEdif is a significant determinant of LV hypertrophy, LV geometry, and diastolic function, and therefore, evaluation of the MEdif combined with the average of morning and evening SBP might be useful in the early stage assessment of hypertensive patients.