The outcome in a series of 23 cases of tetralogy of Fallot diagnosed prenatally was compared to published figures for this condition when (a) identified postnatally or (b) treated surgically. There was a marked difference in the survival between these groups, with the highest mortality (75%) occurring when the diagnosis had been made prenatally. The high incidence of chromosomal and extracardiac anomalies in this group (60%) largely accounted for the discrepancy. This, and the potential for progressive changes in the anatomy of the defect during pregnancy, must be taken into account by the pediatric cardiologist offering prognosis in early pregnancy.