Objectives To reveal the relationship of brain motion and cerebrospinal fluid (CSF) flow by phase-contrast cine MRI, and to evaluate this technique in differentiating between arachnoid cysts and subarachnoid space enlargement. Methods Using a phase-contrast cine MRI pulse sequence, we measured brain motion and CSF flow during the cardiac cycle in 10 healthy volunteers and 10 patients with MRI-suspected arachnoid cyst or subarachnoid space enlargement. CSF stroke volume curve was illustrated according to flow quantification, and time-signal intensity curve was traced. The two curves were compared. Results This study showed that brain motion was due to the volume difference between arterial and venous blood flow during a cardiac cycle, and thus drives CSF pulsation. Arachnoid cysts and subarachnoid space enlargement carried different curve patterns, demonstrating that phase-contrast MRI and flow quantification can be a useful and reliable technique for non-invasive evaluation of brain motion and CSF flow. Conclusion Arachnoid cysts can be successfully differentiated using phase-contrast cine MRI from subarachnoid space enlargement.