The National Institute of Standards and Technology (NIST) recently established a Vitamin D Metabolites Quality Assurance Program (VitDQAP) in collaboration with the National Institutes of Health (NIH) Office of Dietary Supplements. Participants in the fourth exercise of this program, the Summer 2011 Comparability Study, were asked to use the methodology of their choice to measure concentrations of 25-hydroxyvitamin D in control and study materials distributed by NIST. The study materials consisted of SRM 968e Fat-Soluble Vitamins, Carotenoids, and Cholesterol in Human Serum (Level 1, Level 2, and Level 3) and SRM 968d Fat-Soluble Vitamins, Carotenoids and Cholesterol in Human Serum (Level 1). SRM 2972, which is comprised of separate ethanolic calibration solutions with known concentrations of 25(OH)D2 and 25(OH)D3, was provided as a control material. Participants provided their data to NIST, where it was compiled and evaluated for trueness relative to the NIST value and concordance within the participant community. A report of results was provided to all participants of the exercise, and laboratories were identified by code numbers known only to them. The results from this fourth exercise are reported along with a summary of the analytical methods used.