In a child presenting with malignant phenylketonuria due to dihydrobiopterin synthetase deficiency, the authors studied the cerebrospinal fluid (CSF) homovanillic acid and 5 hydroxyindole acetic acid levels under different circumstances: without treatment; under a treatment with tetrahydrobiopterin used alone at various doses; under a treatment associating BH4, L-dopa, 5 hydroxytryptophan and carbidopa, with increasing doses and varying administration schedules. This biological study compared with clinical evolution, shows clearly the inefficacy, regardless of the doses, of BH4 on the CSF levels of neuromediators and on the neurological status, and the excellent efficacy of neuromediator precursors provided high doses are given, divided in the nyctohemeral period.