Catheter material is considered as one of the main factors of deep venous thrombosis during parenteral nutrition. The effects of five types of catheters (polyethylene, polyurethane, silicone, TFE Teflon, and FEP Teflon) on the vein wall and on the blood flow were compared. Experiments were performed on rabbits in which catheters were inserted into the vena cava. Five series of experiments were performed, each including five animals with the same type of catheter. After 10 days, venograms of the vena cava were performed, animals were then killed and the vena cava was removed with the catheter in situ. The vein and the catheter were macroscopically examined; in cases with thrombosis the clot was weighed. The vein wall and the catheter were examined by conventional microscopy and by electron microscopy, respectively. Obstruction of the venous lumen were significantly more frequent with the rigid catheters than with the soft catheters (p less than 0.001). The most extensive thromboses occurred with the Teflon catheters. The incidence of fibrin sleeves was significantly higher with the soft catheters than with the rigid ones (p less than 0.01). The venographic and macroscopic findings were in agreement in 60-100% of the cases according to the degree of venous obstruction. Catheter surfaces remained unchanged in all cases. The polyethylene catheters induced the most severe intimal inflammatory reactions. Thrombus formation was caused by two mechanisms depending on catheter composition: (1) turbulence caused by the catheter in the blood stream, and (2) platelet aggregation and subsequently, fibrin deposition on the surface of the catheter.