OBJECTIVE: The purpose of this study was to clarify the difference between PSP and PD from the viewpoint of dynamic cerebrospinal fluid (CSF) flow focusing on the midbrain aqueduct. METHODS: Thirty-three PD patients (mean age 69.2±7.9) and 35 PSP patients (mean age 70.5±6.6) were included in this study. CSF flow was calculated by 15 images in an equidistant magnetic resonance imaging (MRI) sequence that was taken throughout a cardiac cycle. RESULTS: Absolute values of the velocity (time points of 2-6 and 12-15, *p<0.05), and the width of the CSF velocity (Vheight) (PSP, 5.1±2.3cm/s; PD, 6.0±1.6cm/s, p<0.05) effectively discriminated PSP from PD patients. On the other hand, conventional MRI measurements discriminated well the midbrain aqueduct area (Area) (PSP, 7.7±2.6mm(2); PD, 5.4±1.8mm(2), p<0.01). Two cutoff value lines (Vheight: 4.75, Area: 5.77) of the ROC curve analysis established two areas for discriminating PSP from PD. CONCLUSION: In the present dynamic CSF flow study, it was newly found that mean velocity of each time point and Vheight showed a more significant decline in PSP than in PD patients, providing a sensitive biomarker for differentiating them. The combination of Vheight and Area could further discriminate PSP from PD patients.