This is an open label, single center, randomized, active comparator controlled study, comparing the effects of vitamin D replacement using oral ergocalciferol versus paricalcitol on parathyroid hormone (PTH) levels in patients with stage 3 and 4 CKD and vitamin D deficiency or insufficiency. The purpose of this study is to determine which of these two approaches is more successful.

This study will enroll chronic kidney disease patients, stage 3 and 4, who have low serum vitamin D levels as defined by K DOQI Guidelines. Patients will be randomized to receive oral daily ergocalciferol or paricalcitol. A total of 80 patients will be enrolled, 40 in the cholecalciferol group and 40 in the paricalcitol group. Outcomes will be assessed as proportion of patients achieving pre-defined changes from baseline and as absolute changes from baseline values. Baseline values will be determined from averaging two consecutive measurements of the variables of interest prior to randomization.

Inclusion Criteria:

1. Male or female chronic kidney disease patients at least 18 years of age.
2. Patients should be able and willing to provide written informed consent and HIPAA Authorization.
3. Chronic kidney disease stage 3 or 4.
4. Plasma intactPTH level above 75pg/ml.
5. Serum 25 OHD level below 30ng/ml.
6. Negative pregnancy test unless 2 years postmenopausal or with a documented tubal ligation or total hysterectomy.
7. Patients need to be off of any Vitamin analogues or preparations for 4 weeks prior to screening.

Exclusion Criteria:

1. Women who are breast feeding, pregnant, capable of becoming pregnant, and not participating in an acceptable form of birth control.
2. Patients currently participating in a clinical trial with another investigational drug or device or who have receive an investigational drug or device within 30 days of enrollment in this study.
3. Major surgery within one month prior to enrollment or planned surgery while patient is in study (other than dialysis vascular access surgery).
4. Patients who have had a malignancy unless they have received curative treatment and have been disease free for 2 years.
5. Patients with a medical status that the PI decides would preclude participation.
6. Patients on hemodialysis or peritoneal dialysis.
7. Patients with a functional renal transplant.
8. Patients with allergies to study drugs.
9. Patients with acute renal failure with the past 12 weeks.
10. Patients with clinically significant gastrointestinal or liver disease.
11. Patients with active granulomatous disease.
12. Patients with urine calcium:creatinine ratio of more than 0.2.
13. Patients who received vitamin D analogs, calcimimetics, or biphosphonates within 4 weeks before screening. Treatment with any one of these medicines during the study is not permitted.