Two female siblings were treated for acute neonatal hyperammonaemia due to complete carbamyl phosphate synthetase I deficiency. The first child was detected clinically at 65 hours of age and therapy started at 79 hours. The second child was followed from birth and therapy started at 5 hours of age. The extrapolated rate of increase of blood ammonia, in the first hours of life before therapy started, was 19 μmol L-1 h-1 in both babies. Peak blood ammonia level was 2235 μmol/L in the first (clinically detected) child and 271 μmol/L in the second (prospectively followed) child. The second child became symptomatic at 3 hours of age when blood ammonia level was as low as 90 μmol/L, whereas blood ammonia levels above 100 μmol/L caused no symptoms during recovery. The child detected clinically required haemodialysis and peritoneal dialysis to treat the hyperammonaemia. In the prospectively treated child, early therapy with intravenous sodium benzoate and sodium phenylacetate slowed the rate of increase in blood ammonia level, but this therapy did not prevent the need for peritoneal dialysis.