To evaluate the efficacy of long-term treatment of individually optimized doses of olmesartan medoxomil compared to nitrendipine in elderly and very elderly patients with isolated systolic hypertension. The study hypothesis is non-inferiority of olmesartan medoxomil compared to nitrendipine in lowering mean sitting systolic blood pressure after 12 weeks of treatment compared to baseline. The study duration is up to 32 weeks, including a 24-week treatment period. After 12 weeks hydrochlorothiazide may be added to control blood pressure. Efficacy and safety measurements are carried out at up to 15 visits during the trial period.