The interior and exterior surfaces of four plastic, 18-gauge intravenous catheters (Abbocath-T, Angiocath, Cathlon IV and Quik-Cath) were examined to identify physical characteristics that could possibly contribute to clinical thrombogenic activity. For each brand, the surface characteristics of one unused catheter (control) and a catheter placed in the left-cephalic vein of an orthopedic surgery patient for 48 hours were compared by scanning electron microscopy. All catheters had surface imperfections. Fibrinous material was found adhering to all used catheters even though exposure of the interior surfaces to blood components was minimal. It is speculated that imperfections on the surfaces of plastic i.v. catheters may contribute to thrombogenic complications in patients.