Olfactory dysfunction is a common manifestation of Parkinson’s disease and a feature of early Parkinson’s disease. The phenomenon has been noted as early as more than 30 years ago. Since olfactory dysfunction can appear before the onset of motor symptoms in Parkinson’s disease, this has made olfactory testing one of the screening items for early Parkinson’s disease. Olfactory testing is highly sensitive in the diagnosis of Parkinson’s disease because it is very rare to find Parkinson’s disease patients with normal olfactory tests (except in women with predominant tremor). Conversely, new research has found that 7% of patients with primary hyposmia (those with just hyposmia, without Parkinson’s disease manifestations) eventually develop clinical Parkinson’s disease after 4 years of follow-up. Recent studies have shown that Parkinson’s disease olfactory impairment is selective for odors and that using a specific combination of olfactory tests (banana, licorice, dill pickle) can help diagnose Parkinson’s disease with an accuracy rate of over 75%. The currently known mechanism of olfactory hyposmia in Parkinson’s disease patients may be that there are nuclei of dopamine neurons in the olfactory bulb, which control the sense of smell, and these dopamine neurons function to inhibit excitatory neurotransmission pathways in the olfactory bulb. In the olfactory bulb of Parkinson’s disease patients, these dopamine neurons are significantly more numerous and hyperfunctional than in normal subjects.