EPSDT (early and periodic screening, diagnosis and treatment), a large-scale operational screening program that has generated a tremendous volume of data on the sociodemographic characteristics and health status of Medicaid-eligible children, seems to provide an ideal context within which to evaluate the effectiveness of preventive child care. Concern about health care expenditures generally, and the effectiveness of preventive child health services specifically, lead to the question of whether the impact on the health status of the children served can be measured without significantly adding to the cost of these services with primary data collection. We employ a quasi-experimental research design, using administratively generated data from an operational EPSDT program to estimate program impact on the prevalence of serious abnormalities among the children served. We found that, compared either with themselves across time or with a control group, a representative sample of 1,831 children had almost 30 per cent fewer abnormalities requiring care on rescreening.