Hemodiafiltration as a blood purification method is a combination of hemodialysis and hemofiltration. With this procedure low molecular substances are predominantly cleared by diffusion while middle molecules are cleared mainly by convection. This report shows how hemodiafiltration can be done using readily available equipment: a regular dialysis machine, an "open membrane" and the COTRAL device. Treatment could be reduced to about 2/3 of dialysis time. With a TMP of 600 mmHg a filtration rate of 60 - 70 ml/min at a blood flow of 200 ml/min and a dialysate flow rate of 500 ml/min the following. Clearance values were obtained: Urea 140 ml/min, creatinine 128 ml/min and inulin 76 ml/min. Increasing the blood flow to 300 ml/min and dialysate flow to 1000 ml/min clearances were: urea 192 ml/min, creatinine 161 ml/min and inulin 91 ml/min. During a 3.5 hour treatment period max. 92.4 urea and 6.3 g creatinine and 36 mg B2 microglobulin were extracted. In our opinion this in our hand advantageous procedure deserves further study.