Fluid withdrawal in over-hydrated patients resistant to diuretics was obtained by means of a capillary haemofilter, using the arterio-venous pressure gradient for blood perfusion at a rate of 100 ml/min. The ultrafiltration rate was 200-600 ml/h and could be maintained as long as 48 h without changing the haemofilter. This method, which needs no technical investment, is easy and simple to handle for the physician, bears only a very low risk for the patient, and ensures a negative fluid balance even at a mean blood pressure of only 60 mm Hg.