Ultrafiltration was used during extracorporeal circulation (ECC) with heart-lung machine in 17 critically ill cardiac patients. In ultrafiltration (hemofiltration), water and small molecules (e.g. urea, creatinine and electrolytes), are separated from the blood by hydrostatic pressure generated on the blood side of a semipermeable membrane. The patients had severe water overload for three reasons, viz. congestive heart failure (10), renal failure (6) or iatrogenic extreme hemodilution (1). On average 2090 (800-5700) ml water was filtered off, increasing the hematocrit from 25 to 33%. Three indications for ultrafiltration during ECC and two modes of such treatment are exemplified in three case reports. No negative effect of the treatment was observed. Ultrafiltration during ECC thus may help to improve the postoperative course in patients with severe water overload due to congestive heart failure, renal failure or iatrogenic extreme hemodilution.