This study was designed to track systemically administered mononuclear cells (MNCs) in the ischemic mouse brain using 7 T magnetic resonance imaging (MRI). Splenectomized wild-type mice were subjected to brain ischemia by 30 or 60 min filamentous occlusion of the middle cerebral artery (MCAo) and reperfusion. Spleen-derived MNCs were labeled with very small superparamagnetic iron-oxide particles (VSOP) and transfused into recipient mice 30 min, 8 h, or 24 h after MCAo via the tail vein. High-resolution MRI sequences were designed to monitor the dynamics of brain ischemia and to observe the migration and engraftment of transfused cells into the ischemic brain. T2*-weighted (gradient-echo) hypointense signal changes became apparent at 24-48 h after transfusion, were typically associated with the ischemic lesion border, and could be followed up to 5 weeks after the insult. Such presumed MNC-associated signal changes in MRI were confirmed by histochemical detection of iron (Prussian blue staining) and detection of constitutively expressed green fluorescent protein (GFP) in a subset of animals transfused with MNCs derived from GFP transgenic mice. Taken together, our results demonstrate that brain engraftment of systemically administered mononuclear cells can be visualized non-invasively over time and space using high-resolution MRI.