Measurement of regional cerebral blood flow (rCBF) in 30 patients with Parkinson's disease using single-photon emission computed tomography and 123I-IMP demonstrated that hypoperfusion was relatively severer in the parietal cortex than other cortices before and after a 1-year follow-up period. The decline in the scores of the Mini-Mental State Examination was significantly correlated with the decrease in rCBF in the parietal cortex during the follow-up period. Our findings suggest that the parietal cortex is involved in the cognitive impairment in patients with Parkinson's disease.