Human phenylalanine hydroxylase (PAH) cDNA was applied as a hybridization probe to analyzing the following 8 restriction fragment length polymorphisms (RFLP) in the PAH genes of 80 normal and 28 phenylketonuric Chinese patients: BglII, 3.6 kb/1.7 kb; EcoRI, 17 kb/11 kb; EcoRV, 30 kb/25 kb; HindIII, 4.2 kb/4.0 kb; MspIa, 23 kb/19 kb; MspIb, 4.0 kb/2.2 kb; PvuIIa, 19 kb/6.0 kb and PvuIIb, 11.5 kb/9.1 kb. The frequencies of the above RFLP in normal Chinese and PKU patients are: 0.13, 0.83, 0.77, 0.81, 0.12, 0.04, 0.70, 0.10 and 0.12, 0.93, 0.89, 0.81, 0.04, 0, 0.69, 0.04, respectively. This study reveals significant differences between the frequencies of individual RFLPs when these values are compared with those of the Caucasians. Finally, the detection of RFLPs in the PAH gene in the Chinese population will permit prenatal diagnosis of PKU in China.