We reviewed the prenatal sonographic findings in 11 consecutive fetuses diagnosed as having abdominal situs inversus (stomach on the right) at a single institution. Interruption of the inferior vena cava with azygous continuation was diagnosed by the "double vessel" sign. This sign was considered to be present in nine cases, including all eight fetuses who proved to have the polysplenia syndrome. The only false positive diagnosis of IVC interruption with azygous continuation was made prospectively in one fetus with asplenia. At autopsy, this fetus showed a large left-sided superior vena cava. We conclude that, in combination with cardiac anomalies or situs abnormalities, interruption of the IVC with azygous continuation should suggest a specific diagnosis of a cardiosplenic syndrome, especially polysplenia. This information should be helpful in patient counseling and management.