Sarcosinemia is a rare inborn error of metabolism that is characterised by an increased level of sarcosine (N-methylglycine) in the plasma and urine. The enzymatic block results from a deficiency of sarcosine dehydrogenase (SarDH), a liver mitochondrial matrix enzyme that converts sarcosine into glycine. Although this condition may remain inapparent until later life, it has been reported in rare cases to lead to neurodevelopmental disability. A 19-year-old male with sarcosinemia presented with dystonia, developmental delay and cognitive impairment. Magnetic resonance imaging revealed vermian hypotrophy. A 2-year pharmacological treatment with memantine was negative on the clinical signs. In this case, it was concluded that the metabolic block leading to sarcosinemia was responsible of a pathologic condition with mental deficiency and complex neurological signs. A maternal isodisomy discovered in the vicinity of SarDH gene could contribute to this pathology. Deficit of SarDH may be considered as a differential diagnosis of growth failure during prenatal stages and respiratory failure at birth following a slowly progressive developmental delay.