The echocardiographic findings and clinical courses in 19 fetuses with marked cardiomegaly [heart length, width, area, cardiothoracic ratio, heart/thoracic area (CTA) ratio > 2 SD] were reviewed. An abnormal heart was the reason for referral in 16 cases of 19 (84.2%). The mean gestational age was 31.5 weeks at detection. Intracardiac hemodynamics were studied using pulsed, color, and M-Mode color Doppler. Despite the heterogeneous etiology of marked cardiomegaly (5 Ebstein's anomalies, 5 tricuspid valve dysplasias, 3 dilatative cardiomyopathies and 6 miscellaneous) a holosystolic tricuspid regurgitation with right atrial enlargement was found in 17 of the 19 cases (89.4%) as a common pathogenetic feature. A hydrops was found in 10 cases (52.6%). Besides 1 elective legal abortion, a high mortality rate of 83.3% was observed in the group followed up (6 intrauterine and 9 neonatal deaths). Among the cardiac measurements the CTA ratio seemed to correlate with fetal outcome, since all three survivors had the lowest values and all fetuses with CTA ratio > 0.6 showed a lung hypoplasia as a sequel of compression by cardiac enlargement. The presence or the development of hydrops, an obstruction of the right ventricular outflow tract, and the lung hypoplasia could be considered as signs of poor prognosis. A reliable differentiation between a patent pulmonary valve, pulmonary stenosis and pulmonary atresia in severe tricuspid regurgitation using modern Doppler means is difficult as seen in our studied group and in the literature.(ABSTRACT TRUNCATED AT 250 WORDS)