Although strict blood pressure control is essential for the prevention of cardiovascular disease, there are many hypertensive patients whose current hypertension treatments are inadequate. Therefore, we designed a clinical study to evaluate the efficacy achieving a blood pressure reduction by combination therapy with losartan and a low-dose diuretic. In the 10 hypertensive patients treated with candesartan at 8 mg/day for more than 2 months, combination therapy with losartan at 50 mg/day and hydrochlorothiazide (HCTZ) at 12.5 mg/day was conducted for 3 months after the cessation of treatment with candesartan. Either immediately before the onset of combination therapy or 3 months after the treatment, the 24 hour ambulatory blood pressure(ABP) and pulse rate were measured every 30 minutes. Pulse wave velocity(PWV) and homeostasis model assessment insulin resistance index(HOMA-R) with fasting blood were also measured after 3 months. In the treatment with candesartan, the ABP was 134.4 +/- 8.7/88.1 +/- 6.1 mmHg at 24 hours, 139.7 +/- 8.4/91.3 +/- 7.1 mmHg in the daytime, 123.8 +/- 11.6/81.6 +/- 5.9 mmHg in the nighttime, and 139.3 +/- 10.3/92.3 +/- 7.3 mmHg in the early morning(6 : 00-8 : 00). After the switch to the combination therapy with losartan and HCTZ, the blood pressure changed to 126.8 +/- 9.3/81.6 +/- 7.0 mmHg (p < 0.05/p < 0.05) at 24 hours, 130.6 +/- 12.0/84.2 +/- 8.5 mmHg (p < 0.05/p < 0.05) in the daytime, 119.3 +/- 8.7/76.6 +/- 6.5 mmHg (NS/NS) in the nighttime, and 129.7 +/- 11.7/84.0 +/- 9.2 mmHg(p < 0.05/p < 0.05) in the early morning. However, both the changes in heart rate and PWV, and the differences in blood glucose, lipid and HOMA-R were not significant. Nonetheless, the blood pressure control for 24 hours was better using the combination therapy with losartan at 50 mg/day and HCTZ of 12.5 mg/day than in the treatment with candesartan at 8 mg/day alone. Since losartan decreases uric acid unlike other angiotensin receptor blockers, the combination therapy of losartan and a diuretic can be expected to provide one of the best therapies for essential hypertensive patients.