For the last 2 years, a program has been developed to screen all hyperphenylalaninemic babies for tetrahydrobiopterin deficiency, by measurement of pterins in urine. High neopterin and low biopterin levels were found in the urine of a 1-month-old girl. Further investigations confirmed an impaired conversion of neopterin to biopterin. No neurological signs were noted, but, in regard to the laboratory data, neurotransmitter replacement therapy was instituted at 2.5 months of age. The most remarkable feature was a rapid increase in the dietetic phenylalanine tolerance, despite the proof that the child was not able to clear a challenging dose of phenylalanine and the record of unchanged pathologically low excretion of biopterin during a 2 month period.