To assess the utility of MR in third cranial nerve palsy.We reviewed precontrast and postcontrast MR of 50 patients with third cranial nerve palsy.MR demonstrated an appropriate lesion in 32 cases. Of these patients, 6 had brain stem lesions and 15 had involvement of the nerve in the cavernous sinus; lesions of the cisternal segment of the nerve were present in 11 patients, with enhancement of this segment observed in 9 patients. An inflammatory or infiltrative source of the palsy was indicated in 19 of these 32 cases. Of 7 patients with pupillary involvement suggestive clinically of a compressive lesion, 4 demonstrated thickening and enhancement consistent with an infiltrative lesion of the nerve. Eighteen patients with pupil-sparing third cranial nerve palsies and a history of diabetes or vascular disease had normal MR findings, with no enhancement of the third cranial nerve observed.Patients who do not have a history of diabetes or hypertension and in whom a complete or incomplete third cranial nerve palsy develops with or without pupil sparing should undergo MR imaging initially (unless there are clear symptoms or signs of subarachnoid hemorrhage) to exclude the presence of an infiltrative lesion or intraparenchymal process. Patients who have a history of vascular disease and a clinical presentation that is suggestive of an ischemic event may be observed initially, but should undergo imaging if improvement does not occur within 3 months.