 This is a multicenter, randomized, parallel group, placebo controlled study designed to evaluate the efficacy, safety, and tolerability of ezetimibe added to ongoing atorvastatin therapy compared with ongoing atorvastatin treatment alone. This study will involve subjects with primary hypercholesterolemia and coronary heart disease CHD who are currently being treated with atorvastatin and who would benefit from additional reduction in low-density lipoprotein cholesterol LDL-C. 