Objectives: To assess the effectiveness and potential cost of an echocardiographic screening program for congenital heart disease (CHD). Patients and methods: Between 01/11/94 and 28/02/98 there were 9697 deliveries in The Royal Maternity Hospital Belfast. Mothers were randomized before delivery. 4875 infants were allocated to the scan group, while 4822 were assigned to clinical assessment alone. High-risk infants were considered separately. Cases of CHD detected before hospital discharge were documented. The annual cost of screening was estimated and the time to accurate diagnosis in each group was assessed. Results: During the study 124 scan allocated infants and 50 controls were identified as having significant CHD before hospital discharge. With a minimum of 3 years follow-up there were 27 additional late diagnoses in controls and 1 in scanned infants. During a single year of the study the mean time to complete diagnosis was 2 days for scanned cases and 110 in controls. The projected cost of screening for all infants was £ 22/infant for the first year. Conclusions: Adding echocardiography to clinical examination greatly enhances early detection of CHD. Although screening is expensive, once established it may reduce the cost of unnecessary outpatient referrals.