A comparison between the cost of identification and care of patients with phenylketonuria (PKU) and congenital hypothyroidism (CH) 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 interviews with patients' families. The analysis yields a benefit-cost ratio of 6.6 for PKU and 13.8 for CH prophylaxis. However, cost-benefit varies depending on the economic partner, i.e. the patient's family, Social Security or Administration.