Patients with Parkinson's disease (PD) are known to suffer from a variety of non-motor symptoms, such as olfactory dysfunction. In fact, the impaired sense of smell is suggested to precede clinically detectable motor signs by several years. Furthermore, several studies demonstrated that the earliest neuropathological changes in PD brain appeared in the olfactory-related brain regions including olfactory bulb. Such an early involvement of olfactory-related areas can explain the hyposmia as one of early signs in PD. On the other hand, recent studies suggested that severe olfactory dysfunction might be a prodromal symptom of dementia associated with PD. Thus, the smell tests seem to be not only a feasible diagnostic tool for the early PD, but also a potentially useful biomarker for the Parkinson disease dementia (PDD).