The Multicenter Isradipine Diuretic Atherosclerosis Study is a randomized, double-blind, clinical trial designed to compare the effectiveness of isradipine against hydrochlorothiazide in retarding the rate of progression of atherosclerotic lesions in the carotid arteries of hypertensive subjects. Eight hundred hypertensive men and women, 40 years of age or older, with minor plaques, are being recruited at eight clinical centers in the United States and will be followed for three years. Quantification of the atherosclerotic lesions at baseline and semiannually is done using B-mode ultrasonography. The background of the trial and its design features are discussed.