This is a 105-week clinical study in patients with heterozygous familial hypercholesterolemia on intensive Low Density Lipoprotein-cholesterol (LDL-C) lowering therapy intended to assess the affects of MK0524A on carotid intima media thickening using ultrasound compared to patients taking placebo. There will be 12 scheduled clinic visits involving review of medical history, physical exam, vital signs, laboratory testing, ultrasound imaging, and electrocardiograms.

Inclusion Criteria:

• Patients who are 18 to 70 years of age with Heterozygous Familial Hypercholesterolemia and a LDL-C greater than or equal to 100mg/dL and triglycerides less than or equal to 400mg/dL at Visit 1 while on a stable dose of intensive LDL-C lowering therapy

Exclusion Criteria:

• A condition which, in the opinion of the investigator, might pose a risk to the patient or interfere with participating in the study
• Patients with less than 80% drug study compliance
• Patients with chronic medical conditions known to influence serum lipids or lipoproteins or significantly affect the ultrasound acoustic window
• Patients with unstable dose of medications
• Pregnant or lactating women, or women intending to become pregnant are excluded
• Patient with diabetes mellitus that is poorly controlled, or is taking new or recently adjusted antidiabetic pharmacotherapy (with the exception of +/- 10 units of insulin)
• Patients with the following conditions: high grade stenosis (greater than 75%) of the carotid artery, chronic heart failure, uncontrolled/unstable cardiac arrhythmias, unstable hypertension, active or chronic hepatobiliary or hepatic disease, HIV positive, episode of gout (within 1 year)

After a detailed review of pooled carotid IMT data from contemporary studies, the Steering Committee recommended that Merck prematurely stop the study; as designed, the study was significantly underpowered. Efficacy analyses were not performed.