Background and Purpose: Matrix metalloproteinase-9 (MMP-9) activity hasbeen associated with hemorrhagic transformation (HT) in experimental modelsof cerebral ischemia. Our aim was to investigate the relationship between MMP-9 concentrations in blood within 24 hours of stroke onset and subsequent HT of cerebral infarction.Methods: We studied 35 patients with a hemispheric ischemic stroke incomparison with 35 normal subjects . Early CT signs of cerebral infarction wereevaluated on admission. The HT and infarct volume were analyzed from the CTperformed on days 4 through 7. MMP-9 levels were determined by enzymelinked immunosorbent assay in blood samples obtained on admission.Results: HT was observed in 8 patients . A total of 7 patients (20%) receivedanticoagulants before the second CT scan. Severity of stroke , infarction size, use of anticoagulants, frequency of early CT signs of ischemia , and treatment with anticoagulants were significantly higher in the group with HT (P<0.001). Mean plasma MMP-9 concentrations were higher in the HT group ( P<0.001). MMP-9 concentrations ≥90 0 ng/mL had the highest sensitivity (100%), specificity (81.48%), and negative and positive predictive values (100% and 61.53%, respectively), MMP-9 900 ng/mL was associated with HT (odds ratio, 1; 95% confidence interval, 1.21; P<0.001) after adjustment for potential confounders and final infarct volume.Conclusions: High plasma MMP-9 concentration in the acute phase of acerebral infarct is an independent biochemical predictor of HT in all strokesubtypes.