An NHLBI-sponsored randomized, double-masked, placebo-controlled, multicenter clinical trial is underway to test the efficacy of the lipid-lowering agent lovastatin and/or the antithrombotic agent warfarin in slowing the progression of early carotid atherosclerosis--as defined by ultrasonographic intimal-medial arterial wall thickening--in a high-risk, asymptomatic population consisting of 919 men and women aged 40-79 years with moderately elevated serum LDL-cholesterol. The Asymptomatic Carotid Artery Plaque Study's (ACAPS) factorial design permits evaluation of each of the two treatments alone as well as assessment of the treatments in combination with each other over a 2.5- to 3.0-year treatment period. Randomized participants receive either 20-40 mg/day lovastatin or lovastatin placebo and either 1 mg/day (minidose) warfarin or warfarin placebo. All participants were encouraged to take low-dose (81 mg/day) aspirin. The primary outcome is the ultrasonographic measurement of the mean of maximum intimal-medial thickness (IMT) across up to 12 preselected segments in the carotid arteries. The secondary outcome of the trial measures the single maximum IMT measurement among the same preselected carotid artery segments. This report describes the rationale for ACAPS, its design, and some baseline characteristics of the study population.