Continuous renal replacement therapy (CRRT) has been advocated as the treatment of choice in critically ill patients with renal failure. However, to be as effective as intermittent hemodialysis in terms of small molecule clearance, the extracorporeal CRRT circuit must operate continuously, 24 hours a day, day after day. This review examines both patient and extracorporeal factors that may lead to premature clotting within the CRRT circuit, and approaches that may reduce extracorporeal clotting. (C) 1997 by the National Kidney Foundation, Inc.