To try to determine the cause of hyperintensity of T1-weighted MR images that occurred on and after day 7 following transient cerebral ischemia, dynamic changes in T1-weighted images and histology of rats subjected to 20 min of 4-vessel occlusion were observed. T1-weighted images showed no remarkable alteration on days 1 and 3, although high signal intensity in the striatal region, in which the T1 value was significantly lower than the values on days 1 and 3, was observed on day 7. High signal intensity in T1-weighted images indicates low T1 values. Histological observation revealed accumulation of microglia in the striatal region on day 7 by lectin staining. There was a tight correlation between T1 values and number of lectin-positive cells. Microglia had stout processes and hypertrophic cell bodies on day 7, resembling lipid-laden phagocytes. Sudan black B staining showed the presence of many fatty droplets in the striatal region on day 7. Furthermore, double staining with lectin and Sudan black B revealed the presence of fatty droplets in bodies of lectin-positive cells on day 7. These results suggest that hyperintensity of T1-weighted images on day 7 following transient ischemia and reperfusion indicates accumulation and phagocytic activation of microglia. T1-weighted images seem to represent the progression of non-reversible tissue injury after transient ischemia and reperfusion.