An elevated concentration of glycine in the blood.

Autosomal recessive metabolic disorder caused by deficiencies in the mitochondrial glycine cleavage system; an enzyme system with four components: p-, t-, h-, and l-proteins; deficiency of the p-protein is the most prevalent form; clinical features include seizures, hypotonia, apnea, and coma; when the illness presents in childhood there tends to be an associated progressive dementia accompanied by extrapyramidal signs.