Most MR imaging studies confirm the diagnostic value of midbrain atrophy in PSP, but evaluation methods vary widely and include subjective estimation11,13,23 or measurement of the transverse or craniocaudal diameter of the midbrain peduncles.4,15 Therefore, it is not surprising that, in addition to reduced midbrain diameters, normal-appearing midbrains have been reported in PSP.11,13,23 A recent report on MSA, PSP, corticobasal degeneration, and healthy controls confirmed the value of anteroposterior measurement of the midbrain diameter in PSP.24 However, clinicians endeavor to identify and differentiate patients with atypical parkinsonian syndromes from those with typical PD rather than from healthy subjects. Our study, therefore, provides individual values of midbrain diameters in PD, atypical parkinsonian syndromes including MSA-P and PSP, and healthy controls from routine MR imaging and clearly shows that the simple measurement of the midsagittal mesencephalic anteroposterior diameter on routine MR imaging may differentiate PSP from typical PD on an individual basis. Anteroposterior midbrain diameters less than 16 mm strongly argue against the diagnosis of PD, with values less than 14 mm found only in patients with PSP. The observation that midbrain diameters measured by a neuroradiologist blinded to clinical diagnosis clearly allowed patients with PSP to be classified as a distinct group without overlap with PD underlines the value of the National Institute of Neurological Disorders and Stroke criteria for the clinical diagnosis of PSP. Interestingly, there was no correlation between duration or severity of PSP and midbrain diameters, indicating that this measure might also be useful for an early diagnosis of PSP.