Objective To study the clinical significance of 3 dimensional time of flight magnetic resonance angiography(3D-TOF-MRA) for the pathogenesis of hemifacial spasm (HFS) and trigeminal neuralgia(TN).Methods 48 patients with HFS and 46 patients without HFS and 42 patients with TN and 40 patients without TN were examined by MRI and 3D-TOF-MRA by the enhancement of DTPA. Diagnosis of the presence of compressions in the root exit zone(REZ) of facial nerves and trigeminal nerves were done by two radiologists on an independent console. Results (1)In the patients, compression of the REZ of the facial nerves and trigeminal nerves were detected on 45 spastic sides (93.8%,neurovascular on 44 sides and tumor on 1 side) and 36 spastic sides ( 85.7%,neurovascular on 32 sides and tumor on 4 sides ), 8 and 4 on the asymptomatic sides (16.7% and 9.5%, all neurovascular ). In the controls, 4 and 5 sides ( 4.4% and 6.3% ) were found in the compression of the REZ of the facial nerves and trigeminal nerves. ( 2 ) The offending vessels of compression of the REZ of the facial nerves were the anterior inferior cerebellar artery (AICA) in 17 cases ( 38.6% ), the posterior inferior cerebellar artery (PICA) in 12 cases (27.3%), the vertebral artery (VA) in 6 cases (13.6%). The offending vessels of compression of the REZ of the trigeminal nerves were the superior cerebellar artery ( SCA ) in 18 cases ( 56.3% ), the anterior inferior cerebellar artery in 5 cases (15.6%), the difficult identified vessels (DIV) in 4 cases (12.5%). (3)The relative risks of microvascular compressions which cause HFS and TN were 26.6 and 9.84. (4) The compressions of the REZ of the facial nerves and trigeminal nerves were proved in 4 cases (neurovascular 3 cases and tumor 1 case) and 10 cases (neurovascular 6 cases and tumor 4 casee) in the operation.Conclusion MRI and enhanced 3D-TOF-MRA appeare to be the best imaging technology for the pathogenesis of HFS and TN now. The major causes of HFS and TN may be different neurovascular compressions in the REZ of the facial nerves and trigeminal nerves, some cases are caused by tumor compression.