To evaluate the occurrence and estimate the frequency of macrocytosis in Williams-Beuren syndrome (WBS). Complete blood count (CBC) data from 179 subjects with WBS aged 1-69 were collected, with common parameters assessed for trends. Z-transformed mean corpuscular volume (MCV) was compared with each laboratory's reference range as well as with control data from the National Health and Nutrition Examination Survey (NHANES) 2013-2014 data archives. Just over a third (35%) subjects had at least one recorded incidence of macrocytosis. In comparisons of CBC parameters with an expected population mean, MCV and MCH were greater than, while Hct and RDW were lower than, expected values. The distribution of erythrocyte MCV is shifted to the right in WBS compared to controls, as was the mean value. Despite this, anemia was absent, except in a single medically complex WBS subject. Though there was a paucity of data available of variables that could potentially cause an elevated MCV, no obvious etiology could be elucidated. Mild macrocytosis without anemia affects a moderate subset of WBS patients, leading to a rightward shift in the MCV distribution curve. Providers encountering isolated mild macrocytosis in WBS can consider observation over further workup. Copyright © 2019 Elsevier Masson SAS. All rights reserved.