We report the use of continuous arteriovenous hemodiafiltration (CAVHD) in a neonate with severe hyperammonemia due to a urea cycle disorder. We compared the ammonia clearance (C(NH3)) for peritoneal dialysis (PD) and CAVHD. C(NH3) for CAVHD was 7.45 ml/min per m2 at a dialysate flow of 300 ml/h and was 10.55 ml/min per m2 at a dialysate flow rate of 600 ml/h. The mean PD clearance was 2.15 ml/min per m2. Our data suggest that CAVHD is superior to PD for the removal of plasma ammonia. We conclude that CAVHD should be considered a reasonable alternative in the treatment of neonatal hyperammonemia in urea cycle disorders when medical treatment fails.