Inclusion Criteria:

- Must be female.

- Must have pathologically confirmed incident, primary invasive breast cancer.

- Must be awaiting surgical resection.

- Women of child-bearing potential (i.e. women who are pre-menopausal or not surgically
sterile) must use acceptable contraceptive methods (abstinence, intrauterine device
(IUD), or double barrier device) and must have a negative serum or urine pregnancy
test within 1 week prior to beginning treatment on this trial. Contraceptive use
needs to be continued at least 1 month after the trial has ended.

- Must provide informed consent.

- Must be willing to discontinue use of calcium and/or vitamin D supplements other than
multivitamin supplementation.

- Participants must have an ionized serum calcium level within normal limits
(1.19-1.29mmol/L) and a total corrected serum calcium of < 10.2mg/dl.

Exclusion Criteria:

- Subjects with life-threatening conditions that would preclude them from breast cancer
treatment including: chronic cardiac failure, which is unstable despite medication
use; uncontrolled hypertension; uncontrolled diabetes mellitus; or unstable coronary
artery disease.

- Patients with severe metabolic disorders, which includes phenylketonuria (PKU),
homocystinuria, and Fabry's disease, that would preclude them from taking calcitriol.

- Patients with a previous history of any other cancer except non-melanomous skin
cancer within the past 5 years.

- Patients with impaired renal function (CRCL < 60 mL/min) or who had kidney stones
(calcium salt) within the past 5 years.

- Patients with hypercalcemia (corrected serum CA > 10.2 mg/dl) or a history of
hypercalcemia or vitamin D toxicity.

- Patients currently taking calcium supplements or aluminum-based antacids must
immediately discontinue their use if they are to enroll in the study.

- Patients currently taking vitamin D supplements must immediately discontinue their
use if they are to enroll in the study.

- Patients with a known sensitivity to calcitriol.

- Women who are pregnant or lactating.

- Women on antiresorptive drugs (e.g. bisphosphonates) within the past year.

- Women currently using oral contraception.

- Women with malabsorptive syndromes (i.e. cystic fibrosis, chronic pancreatitis) or
taking medications that decrease the absorption of fat soluble vitamins (i.e.
Orlistat, Questran).

- Participants assigned to calcitriol who are routinely taking a multivitamin
supplement may continue the supplement as long as the amount of vitamin D in the
supplement is not in excess of the RDA (recommended daily allowance) of 400 IU or 10
μg. If they are not taking a multivitamin supplement, they will be asked to not
start supplementation while on study.