Early myoclonic encephalopathy is an epileptic syndrome with different etiologies. Nonketotic hyperglycinemia is one cause. We describe two cases of early myoclonic encephalopathy, secondary to nonketotic hyperglycinemia, with fatal evolution in the neonatal period. These two cases may better clarify clinical findings that can be associated with impairment of glycine metabolism. Distinguishing features include agenesis of the corpus callosum in patient 1, and weight loss exceeding 10%, associated with metabolic acidosis, in patient 2. The burst-suppression electroencephalography pattern is relatively common in neonatal encephalopathies, and is frequently associated with seizures. Nonketotic hyperglycinemia is an inborn error of metabolism caused by mutations in genes encoding protein in the mitochondrial glycine cleavage system. The neonatal form is a severe, frequently lethal neurologic disease. When associated with electro-clinical features, progressive lethargy and hypotonia occur in the first days of life, progressing to apnea and often death. Prospective treatment with oral sodium benzoate, the N-methyl-d-aspartate receptor antagonist ketamine, and dextromethorphan can favorably modify the early neonatal course of severe nonketotic hyperglycinemia, but does not prevent poor long-term outcomes.