Central venous thrombosis is a well-recognized complication of plastic catheters used during parenteral nutrition and occurs in up to 33% of cases if examined venographically. Silicone elastomer catheters, in addition to their favorable handling properties, are less thrombogenic than plastic varieties and are now increasingly used for parenteral nutrition. However, their use does not prevent the possibility of catheter-related thrombosis as this study demonstrates. In a consecutive series of 118 silicone elastomer subclavian catheters, 83% of which were used for parenteral nutrition, two instances of clinically apparent catheter-related thrombosis occurred and one of these was complicated by major pulmonary embolism. Catheter malplacement, catheter infection, and coagulation abnormalities were absent. Both catheters were used for parenteral nutrition and were in place for 20 and 28 days, respectively. Twenty-eight consecutive silastic catheters used for parenteral nutrition were studied, prior to removal, by simultaneous bilateral ascending phlebography. One clinically inapparent nonocclusive, catheter-related thrombus was detected which detached during catheter removal. This study suggests an incidence of symptomatic and asymptomatic silastic catheter-related thrombosis to be around 4%. Silicone catheters therefore may reduce but cannot abolish catheter-related thrombosis. Furthermore, when present, catheter-related thrombosis may be a source of major pulmonary thromboembolism.