This study was done to determine whether Simvastatin (a medication commonly used to treat patients with high cholesterol levels in the blood increases blood flow to the kidneys and improves renal function in normal volunteers and patients with impaired renal function secondary to polycystic kidney diseases.

Patients with chronic renal failure:

1. Age 18 or older
2. Chronic renal failure secondary to Autosomal Dominant Polycystic Kidney Disease
3. Serum creatinine ≥ 1.4 but ≤ 2.0 mg/dl
4. Creatinine or iothalamate clearance between 35-70 ml/min per 1.73m2
5. Stable renal function for the three preceding months
6. Blood pressure needs to be adequately controlled (< 140/90 mm Hg), with or without the use of antihypertensive agents. However, ACE inhibitors or AT1 receptor blockers must not be initiated, or have their dose increased during the study period.

Patients with normal renal function:

1. Age 18 or older
2. Normal renal function and diagnosed with Autosomal Dominant Polycystic Kidney Disease
3. Serum creatinine ≤ 1.4
4. Creatinine or iothalamate clearance ≥70 ml/min per 1.73m2
5. Renal function has to be stable for the three preceding months
6. Blood pressure needs to be adequately controlled (< 140/90 mm Hg), with or without the use of antihypertensive agents. However, ACE inhibitors or AT1 receptor blockers must not be initiated, or have their dose increased during the study period.

Exclusion criteria include:

1. Patients using corticosteroids, cytotoxic drugs (alkylating agents, chlorambucil, cyclophosphamide), CellCept, or cyclosporin A therapy
2. Positive hepatitis B surface antigen (HBsAg) or hepatitis C antibody (HCV-Ab)
3. Serum transaminase levels (AST, ALT) 2 times the upper limit of normal
4. Clinically significant medical conditions
5. Pregnant patients, patients who are breast-feeding, or women intending to conceive during the course of the study
6. Presence or suspicion of active infection, recent serious infection or chronic/recurrent viral or bacterial infection
7. Presence of concomitant renal artery stenosis.