Maternal uniparental disomy of chromosome 16 (UPD(16)mat) is defined as the presence of two homologous chromosomes 16 inherited from only the mother. Silver-Russell syndrome (SRS) is characterized by growth failure and dysmorphic features, and molecular etiology is unknown in some of the SRS patients. The prevalence of UPD(16)mat in etiology-unknown SRS patients and phenotypic differences between UPD(16)mat and SRS have been insufficiently documented. In this study, we detected two UPD(16)mat patients (2.1%) in 94 etiology-unknown patients with SRS-phenotype. Both patients exhibited preterm birth and growth failure. We suggest considering genetic testing for UPD(16)mat in SRS-phenotypic patients without known etiology. (By Dr. Masayo Kagami, https://jmg.bmj.com/content/early/2018/10/05/jmedgenet-2018-105463)