Tetrahydrobiopterin deficiency in hyperphenylalaninemic babies has to be rapidly recognized since the disease requires a specific follow-up. Based on specimen collection on filter paper, a simple strategy for the screening of this condition has been used since 1987. Urine pteridine measurement can detect 6-pyruvoyl-tetrahydropterin synthase, GTPcyclohydrolase I and pterin-4a-carbinolamine dehydratase deficiencies and direct enzyme measurement in dried blood sample detects dihydropteridine-reductase deficiency. A total of 1,814 hyperphenylalaninemic patients have been studied and 34 tetrahydrobiopterin deficiencies have been detected. The strategy must commend itself by its convenience and simplicity, and can be use on all babies with hyperphenylalaninemia screened in the neonatal period, whatever their blood phenylalanine level.