Using magnetic resonance (MR) planimetry, both the midbrain-to-pontine area ratio (m/p-ratio) and the MR parkinsonism index (MRPI) have been shown to assist in the differential diagnosis of progressive supranuclear palsy (PSP) from Parkinson's disease (PD) and the Parkinson variant of multiple system atrophy (MSA-P). The aim of this study was to determine the diagnostic accuracy of the MRPI compared with the m/p-ratio in a large cohort of 123 patients with neurodegenerative parkinsonism including patients with PSP, PD, and MSA-P. Patients with PSP had significant higher MRPI values and significant smaller m/p-ratios compared with the other groups with overlapping individual values. Overall predictive accuracy was similar for the m/p-ratio (87.0%) and the MRPI (80.5%) with a predictive accuracy for PSP from MSA-P being significantly better for the MRPI (87.5%) compared with the m/p-ratio (75%) as well as a predictive accuracy for PSP from PD being significantly better for the m/p-ratio (87.6%) compared with the MRPI (77.3%). Both the m/p-ratio and the MRPI may assist the clinical differential diagnosis in neurodegenerative parkinsonism.