Olfactory dysfunction in Parkinson's disease (PD) has been described for more than thirty years and known as one of the commonest non-motor symptoms in PD. Recently, it attracts widespread attention as one of early symptoms preceding motor impairments and a potential good marker for the preclinical diagnosis of PD. However, some previous reports demonstrated that olfactory function was also impaired in other dementing disorders such as Alzheimer's disease and it was not specific for PD. But recent pathological re-evaluation suggests that olfactory dysfunction is strongly associated with Lewy body pathology rather than other pathological hallmarks including neurofibrillary tangles. Thus olfactory tests may improve diagnostic accuracy of PD. Increasing evidence suggests that olfactory dysfunction is one of the most important non-motor symptoms in PD.