Fifty-five subclavian veins with previous indwelling catheters were dissected at autopsy. Two groups of subclavian catheters injected with contrast material during removal were studied by cinefluoroscopy. Group 1 consisted of 25 patients with indwelling Teflon and polyethylene catheters. Group 2 comprised six patients with indwelling graphite-benzakonium chloride-heparin sodium (GBH) bonded polyethylene catheters. The vein dissections and the cinefluoroscopic studies in group 1 revealed that totally circumferential fibrin sleeves consistently and extensively formed on indwelling subclavian catheters. They developed on polyethylene, Teflon, nylon, and siliconized rubber catheters. These fibrin sleeves were noted at dissection on catheters within 24 hours after insertion. Two associated thrombi were discovered. Cinefluoroscopy detected one additional thrombus. These findings are an undesirable potential hazard and concern of thromboembolism is certainly warranted. Methods to eliminate fibrin sleeves seem justified. Evidence suggests that GBH-bonded catheters significantly inhibit fibrin sleeve formation.