The value of magnetic resonance imaging (MRI) in investigation of patients with late-onset epilepsy has not been studied systematically. We evaluated prospectively the usefulness of MRI in 50 patients with late-onset epilepsy in whom a computed tomography (CT) scan was normal (32), did not allow a definitive diagnosis to be made (12), or showed irrelevant lesions (6). Patients were assessed clinically and had an EEG, and CT and MRI scans were reported by one neuroradiologist blinded to clinical and laboratory data. Of the 32 patients with a normal CT scan, MRI was normal in 20, showed irrelevant ischemic lesions in 8, and showed the cause of seizures in 4 patients. Of the 12 patients with nondiagnostic CT, MRI clarified the diagnosis in 5 and was normal in 2 patients. In 6 patients, both scans showed irrelevant ischemic lesions, and in 1 patient MRI showed a relevant additional lesion. The incidence of MRI-detected white matter ischemic lesions was no greater than in an age- and sex-matched group of subjects without seizures. MRI was diagnostic in 32% of the patients with partial seizures and/or focal EEG findings, as compared with 0% of patients without focal features (p less than 0.01). We conclude that MRI is useful in investigation of patients with late-onset epilepsy with focal features.