To contain costs associated with lipid-lowering therapy, the U.S. Department of Defense awarded a contract for the statin drugs to the manufacturers of cerivastatin and simvastatin. Hyperlipidemic patients were supposed to be converted to the preferred statins on or after October 1, 1999. OBJECTIVES: The primary goal of this study was to evaluate the clinical and economic outcomes associated with switching statin therapy of patients from a non-preferred statin (atorvastatin, fluvastatin, lovastatin or pravastatin) to a preferred statin (cerivastatin or simvastatin). METHODS: The study was conducted using retrospective data from two groups of patients: those for whom statin prescription records were available (called "Pharmacy Group"); and a subset of the patients in the Pharmacy Group for whom pre- and post-conversion lipid profiles were available (called "Clinical Group"). RESULTS: For the 181,787 patients in the Pharmacy Group, the median decrease between pre- and post-conversion drug cost per patient per month was $5.35. This translates into a cost avoidance of $11.7 million per year. Medication compliance of patients in the Pharmacy Group remained essentially unchanged from pre-conversion period (0.95) to post-conversion period (0.97). Out of the 1,552 patients in the Clinical Group for whom information on National Cholesterol Education Program goal status was available, 56.8 percent were at goal before the switch and 59.1 percent were at goal after the switch. LDL cholesterol decreased by 5.1 percent, HDL cholesterol increased by 5.8 percent, triglycerides increased by 2.1 percent, and total cholesterol decreased by 0.8 percent. CONCLUSION: The statin therapy interchange program resulted in considerable cost savings to the Department of Defense without clinically important changes in lipid profiles.