Efficacy of Vitamin D in Colorectal Cancer Chemoprevention

Patients will be offered participation in this study at the time of their regularly scheduled visit to the UIC Colorectal Cancer Screening Clinic. Those agreeing will have indicated their understanding that participation will be conditional upon their having 10+ ACF at the time of their screening colonoscopy. If at screening colonoscopy 10+ ACF are found patients will: * undergo 3 endoscopic mucosal biopsies of the distal colon; and * undergo a blood draw from an i.v. already in place for sedative-narcotic administration for serum 25(OH)D3 and serum ionized calcium; and * be given either placebo or calcifediol and instructed to take daily for 6 months. * provide urine for calcium/creatine spot ratio. At 7 and 14 days after the screening colonoscopy patients will be called on the telephone by the clinical research nurse assigned to this study to follow-up and note 25(OH)D3 (calcifediol) toxicity, if any (note that toxicity has not been described except in the case of overdose). Signs specifically looked for will include: headache, increased urination, nausea, vomiting, abdominal pain, weakness, constipation, and anorexia. If present, patient will be advised to present immediately to the UIC GCRC for physical and serological evaluation. At 30, 90 and 120 days the patient will have agreed to present to the GCRC clinic for: * Capsule retrieval/count (80% compliance will be required to remain in study); and * Detailed history and physical, focusing specifically on signs and symptoms of hypercalcemia. * At day 90 only - provide urine for calcium/creatine spot ratio. Evidence on exam, or laboratory, of hypercalcemia will result in an adverse event reporting and immediate patient discharge from this study. Signs specifically looked for include: headache, increased urination, nausea, vomiting, abdominal pain, weakness, constipation, and anorexia. At 180 days the patient will have agreed to undergo: * Repeat endoscopic exam limited to the recto-sigmoid colon, also known as a "flexible sigmoidoscopy"; and * repeat blood draw for serum 25(OH)D3 and serum ionized calcium