OBJECTIVE	To compare the efficacy of combination therapy with sustained-release diltiazem and hydrochlorothiazide ( DTZ SR-HCTZ ) with that of monotherapy with DTZ SR , HCTZ , or placebo in the treatment of essential hypertension ; and to determine whether the addition of a diuretic to diltiazem at apparent optimum doses of each agent significantly enhances their antihypertensive effects .\