This study utilized resting-state fMRI data to evaluate effective (directional) connectivity in newly diagnosed unmedicated patients with early PD to derive a comprehensive picture of the nature of cortico–striatal–thalamic loop connectivity in PD. We found decreased effective connectivity between the key areas in the cortico–striatal–thalamic loops in unmedicated early stage PD patients . These results were mostly lateralized to the hemisphere opposite the predominant side of parkinsonian symtoms. The findings of this study are mainly important, given the fact that, looking at changes in effective connectivity and its relation to different disease related factors may help us better understand the heterogeneity of PD and more accurately target therapeutic interventions (i.e. deep brain stimulation).