Computerized tomographic (CT) studies with longitudinal follow-up of the relationship between supratentorial and infratentorial brain atrophy in Parkinson’s disease (PD) using our new CT-derived brain atrophy index (BAI) are reported. In those patients with Parkinson’s disease, CT analysis demonstrated a significant increase in brain atrophy compared with controls. Infratentorial BAI correlated significantly with supratentorial BAIs and the decrease in the infratentorial BAI correlated with a corresponding decrease in the supratentorial BAI in those with PD. It is postulated that the infratentorial brain atrophy found in patients with PD could well be secondary to supratentorial brain atro