The Multicenter Anti-Atheroma Study (MAAS) is a 2 + 2-year, placebo-controlled trial to evaluate the effect of simvastatin, a 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor, on progression and regression of coronary atherosclerosis in patients with established coronary artery disease. This paper describes the aims, methodology, and baseline data. Patients with at least two coronary segments visibly involved with atherosclerosis, in whom an angiogram was carried out according to the standards required for quantitative analysis, were selected provided that the serum total cholesterol was between 5.5 and 8.0 mmol/L and fasting triglycerides were lower than 4 mmol/L. Between March 1988 and October 1989, 383 eligible patients of both sexes aged 30-67 years were randomized in 11 European clinics. patients received either 20 mg oral simvastatin or placebo daily for 2 years in addition to dietary counseling. The primary outcome measures are the change in the mean absolute width and in the mean of the minimal width of segments analyzed quantitatively by coronary angiography performed before and after 2 and 4 years of trial medication. To this end, at least 5 coronary artery segments are analyzed in each angiogram using matched view. The 2-year analysis was completed on 89% of eligible patients in February 1992. The trial was initially designed with a 2-year treatment period. To allow for the possibility to extend this, the decision was taken to keep all patients on the original medication allocation until all 2-year angiograms had been analyzed. Based on a predefined decision rule, an independent committee then recommended extension of treatment with another 2 years, to be concluded by a third angiogram. Of the patients enrolled initially, 81% continued. Four-year follow-up will be completed late 1993 and final results are expected mid 1994.