We characterized four new mutations in the ornithine transcarbamylase (OTC) gene in three male infants who died from acute neonatal hyperammonemia and one male infant with late onset disease. OTC enzymatic activity was undetectable in the livers of the three neonates, whereas residual enzymatic activity was present in the fourth patient. All 10 exons of the OTC gene were amplified by the polymerase chain reaction (PCR) from genomic DNA of the four patients. The amplified DNA was screened for abnormal gel electrophoretic migration patterns via single-strand conformational polymorphism (SSCP). One patient showed an abnormal SSCP pattern of exon 8 and exon 9, a second patient had an abnormal exon 6, a third had an abnormal exon 9, and the fourth patient revealed an abnormal exon 3. Sequencing of the abnormal exons revealed that the first patient had a deleterious mutation in exon 9 consisting of a G-->T transversion in codon 310 causing a Glu-->stop coding change. The abnormal exon 8 of this patient contained a common polymorphism consisting of an A-->G transition in codon 270 resulting in Gln-->Arg code change. The abnormal exon 6 of the second patient contained an A-->G transition in codon 183 causing a Tyr-->Cys change. Exon 9 of the third patient contained a deletion of a thymine nucleotide (base 882) resulting in a shift of the reading frame and a code for premature termination 28 codons downstream. The fourth patient with a "milder" clinical presentation had an A-->T transversion in exon 3 (codon 88) causing a Lys-->Asn change.