Magnetic resonance (MR) imaging was used to evaluate the relationship of the cisternal portion of the fifth cranial nerve to surrounding vascular structures in six patients with documented trigeminal neuralgia and in 85 asymptomatic patients. MR imaging clearly demonstrated the course of the fifth nerve from its root entry zone (REZ) to the Meckel cave and its relationship to the surrounding vertebrobasilar system. In the six patients with trigeminal neuralgia, the presence of a vascular structure at the REZ of the fifth nerve was identified. In the 85 asymptomatic patients, examination of 170 trigeminal nerves revealed that 30% had contact between a vascular structure and the fifth nerve at the REZ, but only 2% had actual deformity. These results indicate that although neurovascular contact may be asymptomatic, MR demonstration of a vascular structure at the REZ of the fifth cranial nerve in a patient with trigeminal neuralgia may implicate this as the cause of symptoms, which may influence the treatment of choice. Because of the inherent limitations of computed tomography in the visualization of posterior fossa structures, MR imaging should be considered the initial screening procedure in the assessment of patients with trigeminal neuralgia.