The role of radionuclide venography is demonstrated in the evaluation of inferior vena cava (IVC) and iliac vein obstruction during late pregnancy. Eight patients with a strong clinical suspicion of lower extremity venous disease underwent routine supine venogram during their third trimester of pregnancy. Because of abnormal image pattern, the study was repeated in either prone or lateral decubitus position. In two of the eight cases, the abnormal collateral veins were no longer visualized indicating functional obstruction of inferior vena cava by the gravid uterus. The remaining six cases continued to have similar image pattern suggesting true venous obstruction. Thus the modified radionuclide venogram is extremely useful in detecting major venous obstruction of IVC and iliac system during late pregnancy.