Circulating 25-hydroxyvitamin D (25-OHD) is associated with a reduction in risk of some cancers, but its association with prognosis among patients with cancer is poorly understood. In view of the increasing number of cancer survivors in the United States and the high prevalence of vitamin D deficiency among patients with cancer, an evaluation of the role of circulating 25-OHD in prognosis among patients with cancer is essential. We conducted a systematic review of studies published in the following databases - PubMed, OvidSP, BioMed Central, EMBASE, and Scopus till September 2013 using the following search terms: "vitamin D," "25-hydroxyvitamin D," "calcidiol," "cancer," "survival," "mortality," and "prognosis." Our search yielded 1,397 articles. From the 1,397 articles, we identified 26 studies that evaluated the associations of circulating 25-OHD with prognosis among patients with cancer. Evidence suggests that circulating 25-OHD levels may be associated with better prognosis in patients with breast and colorectal cancer, but there is a paucity of information on its association with prognosis in other cancers. This review highlights the need for further studies evaluating the role of vitamin D in prognosis among patients with cancer.