Impaired flow due to thrombi or fibrin sleeves occur frequently in central venous catheters inserted for hemodialysis, leading to inadequate dialysis. Traditional treatment for dysfunctional catheters was to instill urokinase into the catheter and allow it to dwell within the catheter. The removal of urokinase from the clinical market has led to the use of alternate thrombolytics, such as tissue plasminogen activator (tPA). However, the smallest commercially available dose of tPA is 50 mg, which costs about US $930. Reconstitution of this dose of tPA, with use of 2 to 5 mg for endoluminal therapy, and a discard of the remaining amount is both wasteful and very costly. We have demonstrated that reconstituted tPA, frozen at -40°C in small aliquots immediately after reconstitution, retains its in vitro activity for a period of up to 22 weeks. However, there is little evidence demonstrating whether frozen tPA retains in vivo thrombolytic activity. To address this issue, we performed a retrospective review of all uses of reconstituted, frozen tPA at our center. Catheters providing inadequate flow for dialysis were treated with a 2 mg dose of tPA. Previously reconstituted tPA was thawed, diluted with sufficient saline to match the internal volume of the catheter and left to dwell until the next planned dialysis session. A tPA course was judged to be successful when the subsequent dialysis session was associated with both flow rates of more than 250 mL/minute and a total blood volume processed of more than 60 L. Between 07/1999 and 04/2000, 23 patients received a total of 42 tPA installations. Dwell times varied between 2 hours and 3 days (median 2 days). Eleven (26%) of the installations met our criteria for success. Of the remaining installations, in 26 (62%) cases dialysis could be completed, and in 5 cases (12%) dialysis could not be completed. No patients developed symptoms suggestive of septicemia attributable to the tPA therapy. Our results suggest to us that; (a) reconstituted, frozen tPA retains in vivo thrombolytic activity, (b) published success rates for the use of a 2 mg tPA dose are likely over-optimistic, and (c) higher doses of tPA should be investigated to determine if they produce a higher rate of successful dialysis.