9121 Background: Vitamin D (D) is a fat soluble vitamin/hormone involved in calcium homeostasis. In vitro, animal model and clinical studies show that D affects immunomodulation, apoptosis and cellular differentiation in addition to skeletal effects. D is obtained primarily through UV exposure (sunlight) or through fortified foods (milk). A recent IOM report stated most people have sufficient vitamin D levels and that supplementation is not necessary. D testing and results of serum D levels at MSKCC were evaluated to characterize the D status of MSKCC patients. METHODS A search was performed for MSKCC patients with for whom serum vitamin D 25-hydroxy levels evaluated were taken from January 1, 2003 to June 31, 2010. Demographic information including age, race, BMI and survival status also were obtained. RESULTS There were 23,909 serum 25-hydroxy vitamin D tests ordered from January 1, 2003 through June 30, 2010 for 12,817 unique individuals. The largest volume of tests (57%) was performed in breast cancer patients. The next largest volume was for colorectal cancer patients (9%), followed by leukemia (7%) and lymphoma (7%). Only 311 (1.3%) of the patients with endometrial cancer had D levels obtained. 75.8% of patients were deficient in vitamin D as defined by serum levels <30 ng/ml. The median vitamin D level was 24.6 ng/ml. In 2003 there were 396 D tests ordered; 846 D tests in 2006; 1711 D tests in 2007; 3928 D tests in 2008 and 8133 D tests in 2009. The majority of patients (53%) did not have serial D testing. CONCLUSIONS The vast majority of patients who had their serum vitamin D level monitored were deficient. An exponential increase in the volume of vitamin D tests ordered over the past 3 years was seen. Most patients are D deficient (defined by serum 25-hydroxy D < 30 ng/ml). A recent IOM consensus statement described most people as sufficient in D, however our study does not support this conclusion. Whether treating D deficiency would improve patient outcome remains to be determined. D status should be considered for all patients deficiency corrected. Clinical trials evaluating D repletion and survival are warranted.