We report on a pair of twins with trisomy 12p diagnosed postnatally. The girls were referred for dysmorphism and global developmental delay and have been followed from 10 months of age. They have different levels of mosaicism for both buccal cells and lymphocytes. Although their phenotypic features were similar, there were different degrees of severity which correlate with the different levels of mosaicism. © 2013 Wiley Periodicals, Inc.