The objective of this study was to review the echocardiographic findings and autopsy correlates of pulmonary atresia with an intact ventricular septum when diagnosed during fetal life. All cases with this lesion (nearly 60) seen in the Perinatal Cardiology unit of Guy's Hospital since 1983 are part of this study. More than half of the cases were detected within the last two years, and all bar three were referred because of suspicion of a cardiac malformation detected on routine obstetric scanning. The cases were divided into two distinct groups. On the one hand were those with dilatation of the chambers of the right heart. These had a uniformly poor prognosis. Severe regurgitation of the tricuspid valve was a characteristic feature. On the other hand were the cases with cavitary hypoplasia due to mural overgrowth. Direct measurements of pressure in one such case revealed suprasystemic values in the right ventricle. Although contemplated, fetal intervention proved impossible. Autopsy revealed cases with either valvar or infundibular atresia. In most cases diagnosed prior to 24 weeks of gestation, mothers opted for termination of pregnancy. All cases with dilatation of the right ventricle died. Of pregnancies which continued, only 30% of children are still alive, all but one being less than two years of age at present. The use of cross-sectional and Doppler echocardiography permits the accurate diagnosis of pulmonary atresia with an intact ventricular septum during fetal life. Autopsied cases showed strong correlation with the echocardiographic findings. The poor prognosis in most cases warrants consideration in the future of intervention during fetal life.