Abstract : The Anglo-Scandinavian Cardiac Outcomes Trial--Lipid Lowering Arm (ASCOT-LLA), performed in hypertensive patients with coronary risk and the Collaborative AtoRvastatin Diabetes Study (CARDS), performed in diabetic patients, also at coronary risk, were prematurely stopped, at the special request of the Data Safety Monitoring Board. Indeed, an interim analysis demonstrated the efficacy of atorvastatin, at a daily dosage of 10 mg as compared to placebo, in the prevention of major cardiovascular events, so that it appeared unethical to continue the study until the end. At the glance of these observations, the ethical and statistical aspects of such prevention clinical trials are discussed, in particular when the premature interruption of the ongoing study appears mandatory, with respect of the main rules of evidence-based medicine