Exon 3 termination mutation of phenylalanine hydroxylase (PAH) gene, the only identified one causing classical phenylketonuria (PKU) in Chinese, was detected in fourteen PKU children from Xi'an. The genomic DNA from these patients was amplified by polymerase chain reaction(PCR) and dot hybridied with specific oligonucleotide probes. This mutation is not present in any of these affected children, which indicates that phenylketonuria in Chinese may be caused by other mutations in phenylalanine hydroxylase locus. PCR amplification combining with oligonucleotide dot hybridization is technically feasible for prenatal diagnosis and carrier screening for PKU.