Tricuspid valve dysplasia or displacement in intrauterine life. In 450 cases of structural heart disease diagnosed prenatally, 38 fetuses (8.5%) had either a dysplastic or a displaced tricuspid valve. The tricuspid valve was dysplastic in 22 fetuses, all of which had evidence of tricuspid regurgitation resulting in right atrial dilation and increased cardiothoracic ratio. An associated abnormality of the pulmonary valve occurred in 16 fetuses. The remaining 16 fetuses had Ebstein's malformation, 14 with evidence of tricuspid incompetence at presentation and 10 with an associated abnormality of the pulmonary valve. Of the 38 cases, the pregnancy was interrupted in 17, spontaneous intrauterine fetal death occurred in 8, 11 infants died postnatally and 2 infants are still alive; additional abnormalities were found in 8 cases (chromosomal anomalies in 2, ventricular septal defects in 2, corrected transposition in 2, the Chiari malformation in 2, supraventricular tachycardia in 1 case and coarctation of the aorta in 1). Fetuses with severe abnormalities are selected for fetal echocardiography by the four chamber screening program and a high rate of natural loss both in intrauterine life and immediately after birth was observed in the 21 cases in which pregnancy was continued. This would explain the higher incidence of tricuspid valve disease in our prenatal compared with postnatal series. Although increased cardiothoracic ratio and associated lesions of the right ventricular outflow tract contribute to the poor outcome in the cases detected prenatally, the absence of these features does not always indicate a good prognosis because progression of disease can occur with advancing gestational age. No absolute measurement or single echocardiographic feature emerged as a consistent predictive factor of prognosis. 