Although the therapeutic response to angiotensin II receptor blockers (ARBs) is assumed impaired in patients with low-renin hypertension, data are scarce about the association between levels of plasma renin activity (PRA) before treatment and response of ambulatory blood pressure (ABP) to ARBs in essential hypertension.We prospectively studied 11 untreated Japanese patients with essential hypertension (3 women and 8 men, mean age: 50 +/- 9 (mean +/- SD) years). After a 4-week drug-free period, telmisartan 20-40 mg was administered orally once daily for 8 weeks. PRA levels were measured before treatment. The first ABP measurement was performed at the end of the drug-free period and the second measurement at the end of the treatment period with telmisartan.Telmisartan significantly decreased the 24-h ABP by 12 +/- 11 mm Hg systolic (P or =0.65 ng/ml/h) than in six patients with lower renin levels (<0.65 ng/ml/h) by 18 mm Hg systolic (P < 0.001) and 11 mm Hg diastolic (P < 0.05). Changes in the 24-h systolic and diastolic ABPs significantly correlated with log PRA before treatment.The ABP lowering effects of telmisartan were dependent on PRA levels before treatment in patients with essential hypertension. Measurement of PRA levels before treatment is thought to be useful for prediction of the ABP lowering effects of telmisartan.