The Simon nitinol filter for percutaneous interruption of the vena cava to prevent pulmonary embolism is currently undergoing a multicenter clinical trial. Preliminary clinical results are reported as work in progress. The results in 44 patients at two centers are analyzed in detail, and major events are reported from 103 patients in 17 centers in the United States during a 10-month period. The filter was successfully inserted via the femoral or jugular route in all patients through a 9-F catheter. The placement procedure was easy and without significant complications. Follow-up studies included plain radiography, ultrasonography, magnetic resonance (MR) imaging, and clinical evaluation. No filter migration or perforation occurred among the 103 patients. Symptomatic occlusions occurred in 7%-9%, comparable to other series, and some asymptomatic occlusions were detected with MR imaging only. The implications of occlusion of the filter are discussed.