PURPOSE: To demonstrate the usefulness of diffusion-weighted MR imaging(DWI) in patients with small acuteinfarction by comparing it with fast spin-echo T2-weighted MR imaging(FSE T2WI). MATERIAL AND METHOD: Weretrospectively analyzed the results of FSE T2WI in 26 consecutive patients who on DWI showed small discretehyperintensities of less than 1.5cm and whose final clinical diagnosis, within one week of clinical attack, wasacute inforction. Lacunar infarcts accounted for 24 cases and 2 small cortical infarcts for two. The onset ofsymptoms occurred within 12 hours (hyperacute stage) in two patients, within 24 hours in seven, within 3 days innine, and within one week in eight. Infarcts as seen on FSE T2WI were categorized as follows : (-) for cases ofimpossible localization with non-visualization ; (+/-) for cases of equivocal localization with faint visualizationand/or poor differentiation from combined chronic infarcts and chronic ischemic changes, or from subarachnoid CSFin cases of cortical infarction ; and (+) for cases of adequate localization with clear visualization andmoderately good differentiation from the associated brain changes, or from subarachnoid CSF in cases of corticalinfarction. These infarcts were analyzed according to the time of onset of symptoms. RESULT: For the localizationof small acute infarctions, DWI was markedly superior to the category(-), moderately superior to the category(+/-).With regard to the onset of symptoms, DWI was markedly or moderately superior to FSE T2WI in 2/2 (100%) ofhyperacute stage diagnosed within 12 hour of clinical attack, in 4/7(57%) diagnosed within 24 hours, in 5/9 (56%)diagnosed within 3 days, and in 1/8 (13%) diagnosed within 1 week(p<0.05). In 12/26 cases(46%), small acuteinfarcts were localized by DWI better than by FSE T2WI. CONCLUSION: Because the signal was unchanged or itsintensity was poor, small infarcts at the acute stage were frequently difficult to localize by FSE T2WI. Inaddition, differentiation of these from combined chronic infarcts and chronic ischemic change was poor. DWI canlocalize small acute infarcts even when the results of FSE T2WI are negative or inconclusive.