This study will compare the efficacy, safety, and effects on LDL particle size of Simvastatin at a dose of 10 mg. per day, niacin at a dose of 0.75 and 1.5 grams per day, and the combination of both drugs in patients with hyper-cholesterolemia and combined hyperlipidemia. Twenty patients will be involved in this study, which will last 23 weeks including a 6-week dietary baseline. Patients will receive one of the above mentioned treatments for a period of 17 weeks.