OBJECTIVE We have previously shown that perfluorocarbon emulsions administered i.v. act as contrast material during sonography by creating moving echoes in veins. Accordingly, we performed a study on rabbits designed to establish the value of perfluorocarbon-enhanced sonography in detecting acute venous thrombi. MATERIALS AND METHODS Constriction of the inferior vena cava (IVC) just above the renal veins was established surgically in 15 rabbits. Thrombus was induced in the IVC in 10 of these; the other five served as controls. Gray-scale and color Doppler sonography were performed before and after the i.v. infusion of 3 ml/kg of perflubron emulsion (formulation AF0102, Alliance Pharmaceutical, San Diego, CA). Sonograms were evaluated for the presence and extent of thrombus. Three additional rabbits with percutaneously induced IVC thrombosis were studied before and after the i.v. administration of 0.2 ml of a preparation of microbubbles of perfluorocarbon gas (formulation PRD002, Alliance Pharmaceutical). In these animals thrombi were confirmed at autopsy. RESULTS Unenhanced sonograms allowed detection of thrombi in the IVC in nine of the 10 animals with surgically induced thrombus but were inaccurate in determining clot extent. Filling the lumen of the IVC with moving echoes created by administration of perflubron emulsion allowed more complete delineation of the size and extent of thrombus and permitted detection of residual venous flow not observed on unenhanced scans. The perfluorocarbon gas microbubbles provided similar benefits. CONCLUSION Our results show that i.v. administration of perflubron emulsion or perfluorocarbon gas microbubbles improves imaging of veins on sonography and may aid in the assessment of thrombi.