The routine use of molecular karyotyping in the evaluation of patients with idiopathic developmental delay with/without dysmorphic features, has led to the delineation of several submicroscopic deletion/duplication syndromes. De novo copy number variations are often presumed to be pathogenic and inherited ones from a healthy parent likely to be not relevant for the phenotype. However, it is difficult to draw such a conclusion for an inherited copy number variation not known to be a common variation. We report on a child with developmental delay, seizures, microcephaly, hypotonia, unusual stereotypical movements, and changes in the white matter who inherited a 17q12 tandem duplication of ∼1.4 Mb from his healthy father. Copy number variations in this chromosomal region are thought to be pathogenic and associated with various phenotypes including developmental delay, growth retardation, seizures, renal disease, and diabetes mellitus. We review all reported cases with 17q12 duplication and discuss the novelty of the phenotype in the present case. We also share our thoughts on submicroscopic complexity that may underlie, at least in part, the wide range of phenotypes in patients with 17q12 duplication. © 2013 Wiley Periodicals, Inc.