Previous studies have shown the usefulness of divalent cobalt isotopes to visualize cerebral damage after stroke. The site of accumulation of cobalt ion is unknown but may be explained by neuronal influx, analogous to that of calcium ion. Additionally, uptake may be due to infiltrating leukocytes or protein-bound cobalt. The aims of this study were to compare 57Co-SPECT with leukocyte SPECT and to compare the SPECT findings with clinical outcome as scored by the Orgogozo scale. METHODS Ten patients with a CT scan positive for middle cerebral artery infarcts were included in the study (7 men, 3 women; mean age 70 yr). Technetium-99m leukocyte and cobalt-SPECT (interval 2-4 days) were made with a double-headed gamma camera, after the injection of 10-15 mCi 99mTc-HMPAO-labeled leukocytes and 0.4 mCi 57Co, respectively. Scans were performed within 5-30 days after onset of the first symptoms. Regions of interest (ROI) containing the area of infarction in the slices displaying enhanced radioactivity or the middle cerebral artery (MCA) region in four successive slices were defined for calculating enhancement ratios. The 99mTc leukocyte enhancement ratio (LER) and cobalt enhancement ratio (CER) were defined as the quotient of radioactivity in the ROI and an identical contralateral ROI. The MCA stroke-scale according to Orgogozo was used to assess neurological deficits at the time of scanning and discharge. RESULTS Cobalt-57 and 99mTc-HMPAO showed uptake in the infarcted brain area in five patients; the quantitative uptake in the infarcted brain area of the two tracers correlated significantly (p < 0.05). Both the LER and the CER correlated significantly (p < 0.05) with the Orgogozo score at the time of scanning. Only the LER correlated significantly (p < 0.05) with the Orgogozo score at discharge. CONCLUSIONS Uptake of cobalt and leukocytes in the peri-infarct tissue suggests that 57Co may visualize a component of the inflammatory response. Divalent 57Co may be convenient to predict clinical prognosis after stroke.