This study is a double-blind, placebo-controlled trial of vitamin D and/or calcium supplementation for the prevention of large bowel adenomas. Subjects will be recruited from 11 Study Centers in North America. Eligible subjects will have had at least one large bowel adenoma removed in the 4 months prior to study entry and no remaining polyps in the bowel after complete colonoscopic examination. Participants will be randomized in a partial 2 x 2 factorial design to vitamin D (1000 IU/day), calcium carbonate (1200 mg elemental calcium/day), both agents, or placebo only (Full Factorial randomization). Women who decline to forego calcium supplementation will be randomized only to calcium alone or to calcium plus vitamin D (Two Arm randomization). Randomization will be stratified by gender, study center of recruitment, and anticipated follow-up interval (see below), and will be conducted separately for female subjects randomized only to vitamin D. We anticipate enrolling up to 3000 participants to reach a total of up to 2400 randomized subjects. As safety measures, blood levels of calcium, creatinine, and 25-(OH)-vitamin D will be obtained at baseline and 1 year after randomization, as well as 3 years after randomization for subjects with a 5-year surveillance cycle. Every six months after randomization subjects will complete a questionnaire regarding compliance with study agents, use of medications and vitamin/mineral supplements, illnesses, hospitalizations, and dietary intake of calcium and vitamin D. The primary endpoint of the study will be new adenomas detected on follow-up colonoscopy. These examinations are scheduled to occur either 3 years or 5 years after the qualifying examination, depending on the follow-up interval recommended by each patient's endoscopist. Some patients may, for medical reasons, have a colonoscopy at a time other than 3 or 5 years after the qualifying examination. Information from these exams will be included in analyses where appropriate. In the primary analyses, the occurrence of new adenomas in the interval between randomization and the follow-up exam will be compared between subjects randomized to vitamin D (with or without calcium) versus those randomized to no vitamin D (with or without calcium), between subjects randomized to calcium (with or without vitamin D) versus those randomized to no calcium (with or without vitamin D; excluding women electing to receive calcium who therefore cannot participate in the calcium component of the study), and between those randomized to calcium plus vitamin D versus those randomized to calcium alone. In secondary analyses, we will examine the impact of baseline vitamin D levels and vitamin D receptor (VDR) polymorphisms on the vitamin D effects. Effects on advanced adenomas will also be assessed as a secondary outcome. Participants will be invited to participate in an optional Observational Follow Up phase of the study that will begin following the end of treatment. In this phase of the study, subjects will continue to be followed on an observational basis (no study treatment) with annual questionnaires until the time of a subsequent colonoscopy that is at least three years from the follow up colonoscopy at which study treatment was ended. We will examine the occurrence of new adenomas in the interval between the colonoscopy exam at the end of study treatment and the exam at the end of observational follow up period.