A water-soluble green coffee bean extract (GCE) has been shown to be effective against hypertension in both spontaneously hypertensive rats and humans. This multicenter, randomized, double-blind, placebo-controlled, parallel group study evaluated the dose-response relationship of GCE in 117 male volunteers with mild hypertension. Subjects were randomized into four groups: a placebo and three drug groups that received 46 mg, 93 mg, or 185 mg of GCE once a day. After 28 days, systolic blood pressure (SBP) in the placebo, 46 mg, 93 mg, and 185 mg groups was reduced by -1.3+/-3.0 mmHg, -3.2+/-4.6 mmHg, -4.7+/-4.5 mmHg, and -5.6+/-4.2 mmHg from the baseline, respectively. The decreases in SBP in the 93 mg group (p<0.05) and the 185 mg group (p<0.01) were statistically significant compared with the placebo group. Diastolic blood pressure (DBP) in the placebo, 46 mg, 93 mg, and 185 mg groups was reduced by -0.8+/-3.1 mmHg, -2.9+/-2.9 mmHg, -3.2+/-3.2 mmHg, and -3.9+/-2.8 mmHg from the baseline, respectively, and significant effects were observed in the 93 mg group (p<0.05) and the 185 mg group (p<0.01) compared with the placebo group. Both blood pressures were significantly reduced in a dose-related manner by GCE (p<0.001). Adverse effects caused by GCE were not observed. The results suggested that daily use of GCE has a blood pressure-lowering effect in patients with mild hypertension.