Pulmonary embolism (PE) from sources superior to the right atrium has been documented in several reports, but mechanical device protection has not been advised because of the morbidity associated with occlusion of the superior vena cava (SVC). Mechanical protection must maintain caval patency, protect against recurrent PE, and be safe. Since the Greenfield filter has fulfilled these criteria when placed in the inferior vena cava (IVC), we tested its suitability for use in the SVC in 11 dogs. Thrombus was harvested from phenolized segments of infrarenal vena cava and embolized into filters positioned in the SVC through the jugular or brachiocephalic vein under fluoroscopic guidance. Central venous pressure (CVP) was monitored before and after embolization. The dogs were allowed to recover and were followed up with monthly vena caval contrast studies and CVP measurements. After 3 months the animals were put to death and autopsies performed to determine the size of any residual thrombus and to examine the pulmonary arteries for signs of PE. Ten dogs had correct placement of the filter in the SVC; there were no perforations and no episodes of PE. CVP rose slightly after embolization into the filter but caval patency was maintained in all animals. Resolution of filter-entrapped thrombi occurred in all animals (mean initial weight = 1.02 gm, mean final weights = 0.11 gm, p less than 0.05). In the eleventh animal the filter was discharged high, with one leg perforating the brachiocephalic vein. No hemorrhage or distal embolism occurred.(ABSTRACT TRUNCATED AT 250 WORDS)