The objective of our study was to evaluate the frequency of treatable inborn errors of metabolism (IEM) in a clinical sample of Mexican children and adolescents with neurodevelopmental disorders (NDD). Amino acids and acylcarnitines in blood samples of 51 unrelated children and adolescents were analyzed by tandem mass spectrometry to detect treatable IEM of small molecules. One patient with isovaleric acidemia and autism spectrum disorder (ASD) and another with beta-ketothiolase deficiency and ASD/intellectual disability/attention-deficit/hyperactivity disorder (ADHD) were diagnosed, indicating an IEM frequency of 3.9% (1:26 subjects). The high frequency of treatable IEM indicates the need to perform a minimum metabolic screening as part of the diagnostic approach for patient with NDD, particularly when newborn screening programs are limited to a few disorders.