Quantitative cine-mode magnetic resonance imaging of the craniocervical junction was performed in 17 patients with a Chiari I malformation to evaluate cerebrospinal fluid (CSF) dynamics, including 8 patients who underwent surgery. The cine-mode magnetic resonance images of these patients were compared with those of 12 normal pediatric and adult subjects. The craniocervical junction was imaged by 16 cardiac-gated velocity-encoded images arranged in a cine loop. These images allowed the measurement of both the magnitude and direction of CSF velocity. Velocity measurements were made in four regions of interest--the foramen Magendie, the foramen magnum, and ventral and dorsal to the spinal cord at C2--and were plotted in relation to the cardiac cycle to produce a CSF velocity profile. All patients who underwent surgery had the same procedure: a posterior fossa craniectomy with C1 laminectomy, lysis of arachnoid adhesions, and duraplasty. Normal subjects had unobstructed flow around the craniocervical junction: a short period of cranial CSF flow was followed by a sustained period of caudal CSF flow. Patients with tonsillar herniation of more than 5 mm had obstructed CSF flow, decreased CSF velocity, and shorter periods of caudal CSF flow. These patients also had preferential cranial CSF flow as compared with the controls. Postoperatively, there was a substantial increase in both the velocity of CSF flow and in the period of caudal CSF flow in the foramen magnum. The postoperative changes mirrored the velocity profiles of the normal subjects. These changes in CSF velocity and direction correlated with a more normal-appearing foramen magnum, a reduction in syrinx size, and an improvement in symptoms.