This paper concerns with two autopsied cases of siblings who died from cerebral disturbances. In these patients hyperammonemia developed in the neonatal phase due to carbamyl phosphate synthetase I (CPS I) deficiency. The patient in Case 1 was admitted 2 days after birth because of oliguria and vomiting. Hyperammonemia developed and she died on the 43rd day. In Case 2 hyperammonemia developed from the 2nd day after birth and she expired on the 42nd day. In both cases the diagnosis of CPS I deficiency was established from autopsy findings of the liver.