As a possible prognostic index, plasma levels of amino acids were serially measured for 1 week in 22 neonates with perforative peritonitis including gastric rupture and necrotizing enterocolitis and 28 neonatal surgical patients without infection as a control, who were subjected to total parenteral nutrition with hypertonic glucose and amino acids formulated for neonates. Plasma levels of branched chain amino acids significantly increased on the first day after operation in peritonitis group, whereas plasma levels of phenylalanine and histidine increased on the third postoperative day and tyrosine increased seventh postoperative day. In the peritonitis group, 5 neonates died within 2 weeks after operation (early death group) because of cardiopulmonary failure and 5 neonates died with hyperbilirubinemia from 1 month to 6 months after operation (late death group). The early death group showed increased levels of total amino acids compared with those in the late death group and survivors. The late death group also showed higher levels of total amino acids than survivors. Plasma levels of tyrosine increased in the early and late death groups compared with survivors. These results indicated that serial measurements of plasma amino acids early after operation showed the existence of liver dysfunction in addition to the derangement of amino acid metabolism in the liver and muscle caused by septic insult, which predicted the prognosis of perforative peritonitis in neonates.