The medical importance of an early screening for phenylketonuria (PKU) and congenital hypothyroidism (CH) has been recognized for a long time. However, only rare cost-benefit analysis have been reported. A comparison between the cost of identification and care of PKU and CH patients and the expenditure for the care of untreated retarded patients has been established on the basis of the activity of the Nord-Pas-de-Calais regional screening centre and of patient's families interviews. The present analysis yields a cost-benefit ratio of 6.5 for PKU and 12.2 for CH prophylaxis. However cost-benefit varies depending on the economical partner: patient's family, Social Security or Administration. The present model may also be useful cost-benefit calculation when new tests will be planned to be introduced in the national neonatal screening programme.