Anomalies of venous return due to absence of the ductus venosus have been described in the literature. This is a report of the prenatal diagnosis of two cases of isolated ductus venosus agenesis occurring at 20 and 37 weeks' gestation, confirmed postnatally by color-coded Doppler sonography. In both cases the hepatic veins assumed the function of the ductus venosus without compromising fetal hemodynamics or causing hydrops. In the first case, a healthy female infant was delivered at term. In the second case, a 46,XY,dup(8) (q21.1q22)[35]/47,idem, + r(8)[15] karyotype was diagnosed by amniocentesis. The male newborn showed facial anomalies, a bell-shaped thorax and increased intermamillary distance. There was muscular hypotonia, delayed psychomotor development and an inspiratory stridor leading to obstructive nocturnal dyspnea. On the basis of our observations in these two cases and previous reports from animal studies, we conclude that absence of the ductus venosus may be compatible with normal fetal development without relevant disturbance of circulation and oxygenation.