Strong adherence to antihypertensive therapy has been shown to reduce the frequency of cardiovascular events by strictly controlling blood pressure. Although calcium channel blockers (CCBs) are among the most popular antihypertensive drugs in Japan, few trials have been conducted using high CCB doses in Japanese patients. In this study, we administered amlodipine 5 mg or 10 mg to patients with hypertension in order to compare the efficacy and tolerability of low and high doses, and measured two surrogate markers of hypertensive target organ damage, i.e., brain natriuretic peptide (BNP) as a risk marker of cardiac overload and microalbuminuria as a measure of renal damage. Seventy-two patients were randomly assigned to either amlodipine 5 mg (n = 35) or 10 mg (n = 37) dose groups. The latter group achieved greater reductions in clinic as well as both morning and evening home BP levels without an increase in pulse rate (the differences between the two groups in clinic/morning/evening systolic BP were 4.7/4.7/5.4 mmHg, and for diastolic BP they were 4.2/3.6/3.8 mmHg). Reductions in BNP and urinary albumin/creatinine ratio (UAR) levels were significantly correlated with the reductions in systolic BP levels (BNP, clinic/morning BP: r = 0.256, p = 0.030/r = 0.330, p = 0.005; UAR, clinic BP: r = 0.316, p = 0.007). In conclusion, the higher dose (10 mg) of amlodipine induced greater reductions in all BP levels than did the lower dose, without increasing the pulse rate. These additional reductions were significantly correlated with reductions in hypertensive cardiac overload, as evaluated by BNP levels, and a reduction in renal damage, as evaluated by microalbuminuria levels. Moreover, a reduction in the microalbuminuria may have occurred concomitant with a reduction in clinic systolic BP level.