The etiology of transient global amnesia (TGA) is not well understood. MR studies, including studies using diffusion-weighted imaging (DWI), have been used to investigate the pathophysiology of TGA, and focal hippocampal lesions have been detected in some studies. The aim of this study was to investigate serial changes in MR images from the patients with TGA. In seven TGA patients, serial MRI scans (from the same day of the・ onset to several days after the, onset of symptoms) using a 1.5-T MR unit were prospectively evaluated. Iri four patients, the duration of TGA was over 12 hr. Three of those patients showed small punctate hippocampal hypersensitivity with decreased ADC values on DW images. These lesions were detected in the postacute phase (a time window of 24 - 48 hr after the onset of symptoms). In follow-up studies performed several days after the onset of symptoms, DWI lesions had disappeared in the subacute phase (7-10 days after the TGA episode). The delayed hippocampal lesion on DW images with 1.5-T MRI in patients with TGA appears to be associated with longer duration of symptoms, to persist for several days and to disappear in the chronic phase.