ECHS1 gene encodes a mitochondrial enzyme, short-chain enoyl-CoA hydratase (SCEH). SCEH is involved in fatty acid oxidation ([Sharpe and McKenzie (2018); Mitochondrial fatty acid oxidation disorders associated with short-chain enoyl-CoA hydratase (ECHS1) deficiency, 7: 46]) and valine catabolism ([Fong and Schulz (1977); Purification and properties of pig heart crotonase and the presence of short chain and long chain enoyl coenzyme A hydratases in pig and guinea pig tissues, 252: 542-547]; [Wanders et al. (2012); Enzymology of the branched-chain amino acid oxidation disorders: The valine pathway, 35: 5-12]), and the dysfunction of SCEH leads to a severe Leigh or Leigh-like Syndrome phenotype in patients ([Haack et al. (2015); Deficiency of ECHS1 causes mitochondrial encephalopathy with cardiac involvement, 2: 492-509]; [Peters et al. (2014); ECHS1 mutations in Leigh disease: A new inborn error of metabolism affecting valine metabolism, 137: 2903-2908]; [Sakai et al. (2015); ECHS1 mutations cause combined respiratory chain deficiency resulting in Leigh syndrome, 36: 232-239]; [Tetreault et al. (2015); Whole-exome sequencing identifies novel ECHS1 mutations in Leigh, 134: 981-991]). This study aims to further describe the ECHS1 deficiency phenotype using medical history questionnaires and standardized tools assessing quality of life and adaptive skills. Our findings in this largest sample of ECHS1 patients in literature to date (n = 13) illustrate a severely disabling condition causing severe developmental delays (n = 11), regression (n = 10), dystonia/hypotonia and movement disorders (n = 13), commonly with symptom onset in infancy (n = 10), classical MRI findings involving the basal ganglia (n = 11), and variability in biochemical profile. Congruent with the medical history, our patients had significantly low composite and domain scores on Vineland Adaptive Behavior Scales, Third Edition. We believe there is an increasing need for better understanding of ECHS1 deficiency with an aim to support the development of transformative genetic-based therapies, driven by the unmet need for therapies for patients with this genetic disease.