In this placebo-controlled trial involving patients with recently diagnosed adenomas, daily supplementation with vitamin D 3 (1000 IU), calcium (1200 mg), or both did not reduce the risk of recurrent colorectal adenomas over 3 to 5 years. Vitamin D, an essential nutrient that is important for bone mineralization and calcium homeostasis, 1 also has effects beyond bone and calcium. Many studies have shown it to be antineoplastic, particularly in the colorectum. In in vitro studies, vitamin D and its analogues have been shown to inhibit proliferation, induce differentiation, inhibit angiogenesis, and promote apoptosis in epithelial tissues. 2 , 3 High vitamin D intake inhibits experimental carcinogenesis, 2 , 3 even in animals that are vitamin D–replete. 4 Observational studies of vitamin D intake 5 – 7 and serum levels of 25-hydroxyvitamin D 8 – 10 have shown inverse associations between these measures and the risk of colorectal . . .