The hidden mortality of transposition of the great arteries and survival advantage provided by prenatal diagnosis.OBJECTIVE:  To describe the sensitivity of fetal anomaly scanning at detecting transposition of the great arteries (TGA) and to investigate whether prenatal detection improves survival. DESIGN:  Retrospective review of survival by comparing those who had an antenatal diagnosis with those who did not. SETTING:  Population-based study in Wessex region over 13 years. POPULATION:  Babies with isolated TGA and an intact ventricular septum. METHODS:  Review of outcomes by comparing those who had an antenatal diagnosis with those who did not. MAIN OUTCOME MEASURES:  Mortality rates in each group. RESULTS:  TGA occurred more commonly in boys than in girls. Using the existing national screening policy, the antenatal detection rate of TGA was only 6.9% over the study period, improving to 25% in the last 4 years. This contrasts with a 40% detection rate when TGA was associated with a ventricular septal defect (VSD). All the babies who had TGA diagnosed antenatally survived through surgery. Of those who were not diagnosed antenatally, two were stillborn, five died before the diagnosis was made and four died after surgery. Although the difference in survival rates between those who were antenatally diagnosed and those who were not is not statistically significant (chi(2)= 3.9; P = 0.11), some of these deaths could have been prevented if a prenatal diagnosis had been made. CONCLUSIONS:  Improved antenatal diagnosis could lead to a significant reduction in the mortality associated with TGA. The current low detection rate of TGA in the UK could be improved by the inclusion of outflow tract views in routine fetal anomaly scans, and we believe that the extra workload is justified.