Cystic fibrosis (C.F.), a congenital lethal disease involving many organs, is responsible of chronic pulmonary disease and maldigestion. At the beginning symptoms can be feeble and diagnosis is often delayed, especially in those cases with an isolated pulmonary expression. It is demonstrated that early diagnosis and immediate prophylaxis of pulmonary infections and maldigestion improves survival. Thus a neonatal screening test is required. Although various attempts have been done, dating from 1968, there is no evidence, up to now, of a real utility of neonatal screening tests in C.F. The only test with a minor frequency of false negatives and positives is the RIA trypsin serum dosage to be executed within 3-5 days of life.