Impairment of olfaction is a characteristic and early feature of Parkinson's disease (PD). Recent data indicate that >95% of patients with PD present with significant olfactory loss. Deficits in sense of smell may precede clinical motor symptoms by years and can be used to assess risk of developing PD in otherwise asymptomatic individuals. This article summarises the available literature on olfactory function in PD, which indicates that olfactory tests improve the diagnostic process in these patients; however, it is still unclear which lesion causes olfactory loss in PD.