Background Vitamin D deficiency is common in the United States. Regardless of whether or not vitamin D deficiency increases the risk of cancer and decreases survival of cancer, the established adverse impact of its deficiency on bone health is of particular concern for cancer patients. The extent of vitamin D deficiency is not well defined in the oncology setting, and there are no standardized protocols for screening and supplementation for individuals found to be deficient in vitamin D. Objective To determine the prevalence of vitamin D deficiency as measured by levels of serum 25-hydroxyvitamin D (25[OH]D) in cancer patients at an outpatient oncology practice. Methods A total of 177 patients representing a range of oncologic diagnoses were tested for 25(OH)D between January 1, 2011 and December 31, 2011. Suboptimal vitamin D levels were defined either as less than 20 ng/mL or less than 30 ng/mL, according to standards proposed by the Institute of Medicine and the Endocrine Society, respectively. Limitations The point of testing was subjective to the clinician. Some patients may have had their vitamin D levels tested and treated elsewhere, therefore that data was not captured. Results At baseline, 18.1% of patients tested had vitamin D levels of less than 20 ng/ml, and 49.1% of patients had vitamin D levels of less than 30 ng/ml. Follow-up rates were low. In all, 54% of patients with 25(OH)D levels of less than 30 ng/ml obtained a second reading, and only 38% of those patients achieved sufficient levels at the second reading. Conclusion Vitamin D deficiency is prevalent in patients with cancer and should be monitored in patients who are at high risk for vitamin D deficiency or poor bone health.