PURPOSE: To determine the usefulness of repeat diffusion-weighted imaging (DWI) during the acute ischemic stroke stage for the prediction of evolving stroke and clinical course. MATERIALS AND METHODS: Fifteen patients with acute ischemic stroke in MCA territory [<24 hours, 5 patients;>24 hours, 10 patients; M:F=9:6; age 28 -75 (mean, 61) years] were involved in this prospective study. All patients underwent initial DWI, follow-up DWI (within two weeks of the first attack) and T2WI (2 -5 months later to assess final infarction territory). The National Institute of Health Stroke Scale (NIHSS) was used for clinical evaluation. 'Evolving stroke' was defined as progression of NIHSS after admission. For statistical analysis, Fisher's exact test was used and a p value<0.05 was considered significant. RESULTS: In six patients (40%), the diagnosis was evolving stroke. In four of these (67%), follow-up DWI showed that the infarction territory was more extensive. Evolving stroke occurred 24 -72 hours after the onset of symptoms. DWI obtained 72 hours after onset showed that one patient had developed new infarction. Patients in whom enlarged infarction territory was seen on follow-up DWI showed progression of NIHSS within three days of onset, while those in whom follow-up DWI demonstrated no change showed an improved NIHSS (p < 0.05). Those who underwent initial DWI within 24 hours of onset showed larger infarction territory on follow-up DWI than those who underwent initial DWI later than this (p < 0.05). CONCLUSION: Repeat DWI during the acute ischemic stroke stage might be useful for the evaluation of evolving stroke.