To assess the relationship between home blood pressure and left ventricular mass, we evaluated cardiac echocardiography in 297 hypertensive subjects (188 men and 109 women; mean age, 62.8±10.3 years) who were treated with amlodipine monotherapy over 1 year (mean dose, 5.5±2.3 mg/day). The morning hypertension group (n =57; 19.2%), who had a morning home systolic blood pressure (HSBP) ≥135 mmHg and an evening HSBP <135 mmHg, had a significantly greater left ventricular mass index (LVMI) concomitant with an increase in the homeostasis model assessment insulin resistance index (HOMA-IR) compared to the good control group (n =174; 58.6%), whose morning and evening HSBP were both <135 mmHg, and had a LVMI roughly equivalent to that of the poor control group (n =63; 21.2%), whose morning and evening HSBP were both ≥135 mmHg. By grouping of subjects according to the difference between morning and evening HSBP (ΔHSBP), subjects with a ΔHSBP≥10 mmHg had a significantly greater LVMI than subjects with a ΔHSBP <10 mmHg. Increases in LVMI in these patients were still significant after adjustment for age, gender, dose of amlodipine, alcohol consumption, body mass index, office systolic blood pressure, and morning and evening HSBP. In a stepwise multivariate regression analysis, ΔHSBP (r² =36.2%, p <0.001), morning HSBP (r² =5.5%, p <0.001), HOMA-IR (r² =1.4%, p =0.016) and age (r² =1.0%, p =0.026) were determined to be significant contributing factors for LVMI. This regression model could explain 44.1% of LVMI variability. These results suggest that morning rise in blood pressure is a dominant predictor of left ventricular hypertrophy. (Hypertens Res 2004; 27: 939-946)