Since olfactory dysfunction is among the first signs of idiopathic Parkinson's disease (PD), olfactory testing may aid in the early of 'preclinical' diagnosis of this disorder. Indeed, the proportion of early-stage PD patients with olfactory dysfunction appears to be greater than the proportion of early-stage PD patients exhibiting some of the cardinal signs of PD. Because olfactory function varies in the general population and declines with age, empirically-based criteria are needed by the clinician to establish whether the degree of olfactory loss observed in a given patient is concordant with the presence of PD. In this study, we present cutoff criteria for the optimal assessment of olfactory dysfunction in the evaluation of PD. Specifically, we present scores for the University of Pennsylvania Smell Identification Test (UPSIT) that best discriminate between PD patients and age-matched controls. Receiver operating characteristic (ROC) curves, based upon sensitivity and specificity estimates, were computed for three age groups (< or = 60 yrs, 61-70 yrs, and > or = 71 yrs) and scores with highest sensitivity and specificity were determined. Sex- and age-related differences in the test scores were observed, with lower scores occurring for men and for the older patient groups.