Partial occlusion of indwelling central venous catheters (CVCs) developed as a clinical problem following the trend to leave CVCs in place for the duration of intravenous therapy, which can last for more than 1 year in some cases. The primary manifestation of partial catheter occlusion is the ability to infuse but not aspirate fluids through an indwelling CVC. There is evidence that the problem is at least partially related to a residue of blood products deposited within some CVCs and implanted ports each time blood is aspirated or infused. Over time, these deposits may act as a ball valve when aspiration from the CVC is attempted while still allowing fluid or drug infusions. A preliminary investigation has indicated that this partial occlusion can be corrected by the use of a fibrinolytic drug to "cleanse" the CVC of residual blood products through lysis, thus restoring full CVC patency. Controlled studies are still needed to determine how often the CVC should be cleansed to prevent buildup of blood products in the indwelling CVC.