The potential impact of prenatal screening for the detection of congenital heart disease (CHD) was assessed by prospective analysis of 428 consecutive infant admissions to a supraregional centre; 28 (6.5%) did not have CHD and were excluded from analysis. Of the 400 cases with CDH, 396 (99%) underwent fetal ultrasonography but scanning was performed only before 18 weeks' gestation in 200 (50%). One hundred and forty nine (37%) of all cardiac abnormalities and 149/283 (53%) of severe abnormalities were considered to be detectable prenatally in a screening echocardiographic four chamber view had this technique been used. Prenatal diagnosis of severe CHD actually occurred in only eight (2%) cases and was after 30 weeks' gestation in all. There were 181/347 (52%) of all mothers and 177/253 (70%) of the subgroup with severe abnormalities who expressed an opinion volunteered their preference for termination of pregnancy if mid-trimester diagnosis had been available. Mid-trimester detection of congenital heart disease rarely occurs at present despite fetal ultrasound scanning in almost all pregnancies. More than half of all severe congenital heart defects seen in infancy are potentially detectable by screening. Major training at primary scan level and modification of the timing of existing fetal anomaly scanning would be required for a screening programme to be effective.