Injection of contrast material through 38 central silicone rubber catheters at the time of withdrawal after a mean catheter life of 55 days (range 5-369) has shown no evidence of occult venous thrombosis despite the presence of a fibrin sleeve around the whole length of the intravascular portion in 37 cases. The presence of this fibrin sleeve appears to have been harmless. The absence of thrombosis suggests that silicone rubber is the material of choice for central venous feeding catheters and that the addition of heparin to the infusion fluid is unnecessary.