OBJECTIVE: To study the characteristics of cerebrospinal fluid dynamics in the midbrain conduction duct and upper cervical segment in patients with Chiari malformation combined with spinal cord cavity and to investigate the mechanism of spinal cord cavity formation. Materials and methods: 10 patients with Chiari malformation combined with spinal cord cavitation were admitted to our department from January 2004 to April 2004, and 10 normal volunteers were in the control group; a retrospective cardiac gated phase contrast magnetic resonance method was applied to quantitatively study the cerebrospinal fluid flow in the midbrain conduction duct, the ventral and dorsal sides of the spinal cord at the level of the inferior cerebellar tonsil, the ventral and dorsal sides of the spinal cord at the level of the cervical 2-3, and the ventral and dorsal sides of the spinal cord at the level of the cervical 5-6 in both groups. The maximum velocity of caudal cerebrospinal fluid flow, the maximum velocity of cephalad flow, the ratio of the maximum velocity of caudal to cephalad flow, the cerebrospinal fluid circulation time (interval between two consecutive caudal flows), the onset of caudal flow, the duration of caudal flow, and their position in the cardiac cycle were measured in the subarachnoid region with the greatest cerebrospinal fluid flow. RESULTS: In all study areas, a transient cephalad flow of cerebrospinal fluid was seen in both the case and control groups during early systole, followed by a definite caudal flow during mid-late systole and early diastole, and then again during late diastole. In our patients, the cerebrospinal fluid circulation time was shorter than normal at all levels (P=0.014 at the level of the midbrain aqueduct; P=0.019 at the level of the inferior cerebellar tonsil; P=0.014 at the level of the cervical 2 and 3 intercalated discs; P=0.022 at the level of the cervical 5 and 6 intercalated discs), and the velocity of caudal (P=0.018) and cephalad flow (P=0.007) of cerebrospinal fluid at the level of the midbrain aqueduct were significantly increased, and the caudal flow velocity of cerebrospinal fluid ventral to the spinal cord at the level of the inferior border of the cerebellar tonsils