It has yet to be established whether substantial reduction of plasma lipids will lead to retardation, and to what extent and how quickly, of diffuse and focal coronary atheroma. The Multicentre Anti-Atheroma Study (MAAS) is a randomised double-blind clinical trial of 381 patients with coronary heart disease assigned to treatment with diet and either simvastatin 20 mg daily or placebo for 4 years. Patients on simvastatin had a 23% reduction in serum cholesterol, a 31% reduction in low-density lipoprotein cholesterol, and a 9% increase in high-density lipoprotein cholesterol compared with placebo over 4 years. Quantitative coronary angiography was done at baseline, and after 2 and 4 years. 167 patients (89%) on placebo and 178 (92%) on simvastatin had baseline and follow-up angiograms. In the placebo group there were reductions in mean lumen diameter (-0.08 mm) and in minimum lumen diameter (-0.13 mm). Treatment effects were +0.06 (95% CI 0.02 to 0.10) and +0.08 mm (0.03 to 0.14) for mean and minimum lumen diameter, respectively (combined p = 0.006). Patients on placebo had an increase in mean diameter stenosis of 3.6% and the treatment effect of simvastatin was -2.6% (-4.4 to -0.8). Treatment effects were observed regardless of diameter stenosis at baseline. On a per-patient basis, angiographic progression occurred less often in the simvastatin group, 41 versus 54 patients; and regression was more frequent, 33 versus 20 patients (combined p = 0.02). Significantly more new lesions and new total occlusions developed in the placebo group, 48 versus 28, and 18 versus 8, respectively. There was no difference in clinical outcome. The numbers of patients who died or had a myocardial infarction were 16 and 14 in the placebo and simvastatin groups, respectively. In the placebo group more patients underwent coronary angioplasty or re-vascularisation, 34 versus 23 on simvastatin. The trial showed that 20 mg simvastatin daily over 4 years reduces hyperlipidaemia and slows progression of diffuse and focal coronary atherosclerosis.