Purpose: To retrospectively evaluate the size of the trigeminal nerve on magnetic resonance (MR) images of patients with unilateral trigeminal nueralgia. Materials and Methods: Institutional review board approval was obtained and informed consent was waived for this HIPAA-complaint study. The sizes of the trigeminal nerves in 31 patients (18 men and 13 women; mean age, 68 years; age range, 44-84 years) with clinically confirmed intractable unilateral trigeminal neuralgia were measured before treatment with gamma knife radiosurgery. Images were analyzed separately by two neuroradiologists who were blinded to the side of the face with symptoms. Coronal projection images were used to determine the diameter and cross-sectional area of the trigeminal nerves at 5 mm from the entry point of the nerve into the pons. Comparisons were made by using a paired t test. Interobserver variability was assessed by using the Pearson correlation coefficient. Results: The mean diameter of the trigeminal nerve on the symptomatic side was significantly smaller than the mean diameter on the asymptomatic side in 30 to 31 patients (2.11 mm ± 0.40 [standard deviation] and 2.62 mm ± 0.56, P < .001, 95% confidence interval: -0.35, -0.67 mm). The mean cross-sectional area on the symptomatic side was significantly smaller than the area on the asymptomatic side in 27 of 31 patients (4.50 mm2 ± 2.19, P < .001, 95% confidence interval: -2.41, -1.16 mm2). Conclusion: The results indicate that trigeminal nerve atrophy can be depicted noninvasively in patients with trigeminal neuralgia.