ABSTRACTThis study seeks to establish a method for opportunistic evaluation of sacral bone mineral density. This is a retrospective review of 109 scans from 109 patients who had renal‐protocol computed tomography (CT) scans performed for any indication during a 3‐month period at a single academic institution in 2014. In the collected CT scans, sacral CT‐attenuation in multiple regions of interest (ROI) was compared to the L1 CT‐attenuation, an internal reference standard, to determine if a correlation existed. The sacral ROI were analyzed to determine regions of higher and lower attenuation. All sacral ROI had strong correlations with lumbar spine attenuation values, and these values became even stronger when transitional vertebrae were excluded. Sacral attenuation values varied predictably by location, and matched relationships were shown by prior volumetric bone mineral density studies. We conclude that sacral CT‐attenuation can be used in opportunistic CT scans to determine sacral bone mineral density. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:160–166, 2017.