A total of 66 vena cava filters was implanted to prevent pulmonary emboli: 33 Mobin-Uddin umbrellas and 33 Kim-Ray-Greenfield devices. Radiologic examinations were available in 59 patients, 37 having one or more follow-up studies. Most of the Mobin-Uddin umbrellas became thrombosed. Although most Kim-Ray-Greenfield devices remained patent, some migrated, and several penetrated the cava wall; this latter finding can be diagnosed on plain films. Neither of these devices is fully safe nor effective in preventing pulmonary emboli; further improvements are needed.