A defect in the synthesis of dihydrobiopterin was detected in an Arab girl, ascertained through high blood phenylalanine level on neonatal screening. An oral loading test with tetrahydrobiopterin (BH4) caused a significant fall in her blood phenylalanine and a rise in tyrosine concentrations. Her blood biopterin levels were low. In urine and cerebrospinal fluid (CSF) very high neopterin and low biopterin levels were observed. A deficiency of metabolites of neurotransmitters, serotonin and dopamine, was observed in CSF and urine. The patient was given replacement therapy of BH4, 5-hydroxytryptophan, and L-dopa with carbidopa starting from the age of 16 to 18 weeks. On this treatment the blood phenylalanine levels dropped to the desired range, while in urine and CSF a satisfactory rise of neurotransmitter metabolites was observed. In spite of this biochemical control, the patient developed neurological symptoms with myoclonic jerks and changes in muscle tone and presented severe cerebral damage with mental retardation. She died suddenly at the age of 38 weeks.