© 2015 WILEY PERIODICALS, INC. Cornelia de Lange syndrome (CdLS) is characterized by facial dysmorphism, growth failure, intellectual disability, limb malformations, and multiple organ involvement. Mutations in five genes, encoding subunits of the cohesin complex (SMC1A, SMC3, RAD21) and its regulators (NIPBL, HDAC8), account for at least 70% of patients with CdLS or CdLS-like phenotypes. To date, only the clinical features from a single CdLS patient with SMC3 mutation has been published. Here, we report the efforts of an international research and clinical collaboration to provide clinical comparison of 16 patients with CdLS-like features caused by mutations in SMC3. Modeling of the mutation effects on protein structure suggests a dominant-negative effect on the multimeric cohesin complex. When compared with typical CdLS, many SMC3-associated phenotypes are also characterized by postnatal microcephaly but with a less distinctive craniofacial appearance, a milder prenatal growth retardation that worsens in childhood, few congenital heart defects, and an absence of limb deficiencies. While most mutations are unique, two unrelated affected individuals shared the same mutation but presented with different phenotypes. This work confirms that de novo SMC3 mutations account for ~1%-2% of CdLS-like phenotypes. Cornelia de Lange syndrome (CdLS) is a multisystem developmental disorder caused by mutation in five genes encoding subunits or regulators of the cohesin complex. To date, only the clinical features of the unique mildly affected CdLS male with SMC3 mutation have been published. Here, we report a series of 16 probands with 15 different intragenic mutations in SMC3 that provide a significant advance in our understanding of the clinical and molecular basis of Cornelia de Lange syndrome and overlapping phenotypes. ; CdLS Foundation of UK and Ireland for their long-term help and support. M.A.D. and I.D.K. are indebted to the USA Cornelia de Lange Syndrome Foundation ; Peer Reviewed