Background: To determine the individual effect of abnormalities in blood pressure (BP) circadian rhythm (nondipping status (NDS), increased morning BP (MBP) or increased MBP surge (MBPS)) on target organ damage (TOD) and which of these is more closely related to TOD in normotensives. Methods and Results: The 24-h ambulatory BP monitoring (ABPM) and echocardiography were performed and urinary albumin excretion (UAE) was measured in 47 dipper (28 women, mean age 45.8 ±9.3) and 32 non-dipper (25 women, mean age 49.1 ±8.3 years) normotensive subjects. The left ventricular mass index (LVMI) was higher in non-dipper group (103.8 ±24.1 vs 91.6 ±23.5 g/m², P=0.03). UAE in non-dipper group was higher, but the difference between the two was not statistically significant (18.9 [10.3, 28.9] vs 14.1 [7.5, 23.8], P=0.11). In multivariate analysis, both LVMI and UAE were affected by NDS and MBP independent of other confounding variables (for LVMI; Coefficient =0.27, P=0.01 and Coefficient =0.37, P=0.001, respectively, and for UAE; Coefficient =0.27, P=0.02 and Coefficient =0.28, P=0.01, respectively). Conclusions: It may be postulated that increased night and MBP are the factors that cause TOD, and it seems reasonable to attempt to restore normal diurnal rhythm of the BP even in normotensive subjects. (Circ J 2009; 73: 899 - 904)