PURPOSE: To evaluate the frequency, characteristics, and clinical significance of transient hyperintensity foci on T1-weighted images (T1WI) in acute disseminated encephalomyelitis (ADEM). MATERIALS AND METHODS: Patients diagnosed with ADEM underwent MR studies at the time of disease onset and every 3 months or more often thereafter. The frequency and appearance timing of abnormal signals including T1WI and their morphological characteristics were evaluated. Relations between patient symptoms and abnormal signals on MRI were also evaluated. RESULTS: Five ADEM patients were included in this study. Linear (n = 2) or nodular (n = 1) T1-hyperintensity foci appeared in 3 patients (60%, 3/5). Locations of T1-hyperintensity foci were both cortical/subcortical region and basal ganglia (n = 1), subcortical region alone (n = 1), and internal capsule (n = 1). Those T1-hyperintensity foci were located within the T2-weighted image (T2WI) and fluid-attenuated inversion recovery (FLAIR) hyperintensity foci on initial MRI. Some T1-hyperintensity foci also showed hyperintensity on diffusion-weighted image (DWI) and contrast enhancement. T1-hyperintensity appeared at 14-43 days (median, 28 days), and disappeared in 2 patients at 91 days and 627 days after disease onset. There were no neurological sequelae remained in any patients. CONCLUSION: T1-hyperintensity foci is not a rare finding (60%) and it can be observed after improvement in symptoms in ADEM.