Aim. To evaluate the efficacy and tolerability of candesartan cilexetil 32 mg in combination with hydrochlorothiazide (HCT) 12.5 mg or 25 mg in hypertensive patients not optimally controlled with candesartan monotherapy. Patients and methods. A total of 3521 patients with treated or untreated hypertension and sitting diastolic blood pressure (DBP) 90-114 mmHg, entered a single-blind run-in phase with candesartan (16 mg for 2 weeks, followed by 32 mg for 6 weeks). At the end of the run-in phase, 1975 patients who still had DBP 90-114 mmHg were randomized to 8 weeks' double-blind treatment with either candesartan 32 mg (n=654), or candesartan-HCT 32/12.5 mg (n=656), or candesartan-HCT 32/25 mg (n=665). Principal results. At randomization, the mean blood pressure was similar in the three treatment groups (approximately 153/97 mmHg). It was reduced during the double-blind treatment phase by 6.1/5.6 mmHg in the candesartan 32 mg group, by 13.0/8.8 mmHg in the candesartan-HCT 32/12.5 mg group, and by 15.5/10.0 mmHg in the candesartan-HCT 32/25 mg group (p < 0.01 for all between treatment comparisons). All study treatments were generally well tolerated. Conclusion. Candesartan-HCT 32/12.5 mg and candesartan-HCT 32/25 mg are highly effective and provide improved blood pressure reduction and blood pressure control relative to candesartan 32 mg monotherapy, with maintained tolerability, in hypertensive patients whose blood pressure is not optimally controlled with candesartan monotherapy. Furthermore, candesartan-HCT 32/25 mg is more effective than candesartan-HCT 32/12.5 mg in this population.