Though the etiology of Parkinson's disease (PD) is unresolved and may be heterogeneous involving both environmental and genetic factors, there are indications that oxidative stress plays an important role in dopaminergic neuronal death. And, it has been reported that inhibition of nitric oxide synthase (NOS) can prevent the destruction of dopaminergic neurons in mammals. To determine if NOS gene polymorphism affects the 5' flanking region that is immediately upstream of the transcription start site lying between the TATA element and CAATT boxes in PD, and differs significantly between patients with PD and normal controls, we studied genetic polymorphism in that region of the neuronal NOS gene in Chinese patients with PD living in Taiwan. The results indicate that the allele size distribution in that region was statistically significantly different between patients with PD and normal.