The treatment of acute renal failure (ARF) in the newborn with hemo- or peritoneal dialysis is technically difficult and may even be contraindicated. As in the adult, continuous arterio-venous hemofiltration (CAVH) may be an alternative therapy. We used CAVH in the treatment of four newborns with ARF of different etiology. Two brachial, one femoral and one umbilical arteries were cannulated as arterial access, while three jugular and one umbilical veins were used as venous return. An Amicon 0.005 m2 Polysulphon Hollow Fiber hemofilter was connected to the patient with shortened pediatric hemodialysis lines. Total blood volume of the extracorporeal circuit was 15 to 22 ml. Before starting the procedure, an initial bolus of heparin was administered to the patient (100 i.u./kg body wt) and a successive continuous heparin administration was provided during the treatment at the rate of 5 to 7 i.u./kg/hr. Hyperalimentation and/or buffer solutions were used as replacement fluids and were administered according to the patient's fluid balance. mean data in the four patients are summarized as follows. The age of the patients ranged from two to 12 days, while the average body weight was about 3 kg. The ultrafiltration rate during the treatment averaged 0.9 ml/min with a plasma flow ranging from 9.8 to 19.6 ml/min. The treatment duration varied from 30 to 86 hrs. The treatment was well tolerated (patients 1 and 2 recovered, and patients 3 and 4 died due to complications unrelated to the treatment). Arterial pressure remained stable during the procedure. Metabolic acidosis, when present, was corrected by increasing the amount of buffer administered. BUN was maintained below 60 mg/dl in three patients.(ABSTRACT TRUNCATED AT 250 WORDS)