Abstract
Phenylketonuria with progressive neurological deterioration is now well known, and has usually been shown to be the result of a defect in biopterin metabolism. This report describes a baby with a progressive neurological disorder present from birth and marked hyperphenylalaninaemia. This responded biochemically, but not clinically, to restriction of phenylalanine in the diet but not to the administration of tetrahydrobiopterin, and no evidence of a defect in biopterin metabolism was detected.