In a series of 12 patients with syringomyelia associated with Chiari's malformation, the authors quantitatively analyzed cerebrospinal fluid (CSF) dynamics in the subarachnoid space of the craniospinal junction, using cine MRI combined with pre-saturation method. In most of subjects, cine MRI revealed (1) decreased or increased maximum velocity of CSF in the caudal direction and (2) disturbed CSF motion in the caudal direction (delayed % cardiac cycle) in the craniospinal junction, strongly suggesting disturbed CSF dynamics in the craniospinal junction because of the tonsilar herniation. Of 12 subjects, 8 patients underwent foramen magnum decompression and 4 underwent syringo-subarachnoid shunt (SS shunt). In the patients who showed marked collapse of syrinx after foramen magnum decompression, follow-up cine MRI revealed the normalization of % cardiac cycle, representing postoperative improvement of CSF dynamics in the craniospinal junction. On the other hand, % cardiac cycle did not improve significantly in the patients who did not show marked collapse of the syrinx or suffered from meningitis after surgery. Significant changes were not observed in the patients who underwent SS shunt. In summary, these results suggested that cine MRI combined with pre-saturation method could detect the pathophysiological changes and evaluate the efficacy of the surgery, especially foramen magnum decompression, in the patients with syringomyelia associated with Chiari's malformation.