Prior to the onset of the cardinal motor features of idiopathic Parkinson's disease (PD), other manifestations of neurodegeneration such as olfactory dysfunction are often apparent. Characterizing these potential biomarkers of preclinical PD is particularly important in identifying individuals who will go on to develop disabling symptoms, and thus be good candidates for new neuroprotective strategies. As shown by the Braak neuropathologic staging of PD, the olfactory system is among the first neuronal populations to display Lewy body pathology. Clinically, loss of smell can be easily tested in the office using several validated techniques and is often helpful to the physician in distinguishing idiopathic PD from other forms of parkinsonism. Recent findings have indicated that a decline in olfaction may be observed in selected at-risk patients, which has significant implications for identifying potential study populations. Ongoing studies of olfactory dysfunction may also reveal potential for use as a medication-independent biomarker of disease progression in addition to use as a biomarker for the diagnosis of PD.