The primary olfactory cortex (POC) responds to both odor-smelling and sniffing. It is not known if there are deficits in the sniffing-related or odor-related functional activities in the POC of early-stage Parkinson’s disease (PD). Here we report significant PD-related deficit in the odor-related POC activation, while the sniffing-related activation was not significantly affected. These results suggest that olfactory deficits in early-stage PD are mainly due to the breakdown of the bottom-up mechanism. In addition, our finding of a negative correlation between the UPDRS-3 score and the odor-related POC activation suggests a surrogate marker for the clinical severity in early-stage PD.