Background and Purpose— Matrix metalloproteinase-9 (MMP-9) activity has been associated with hemorrhagic transformation (HT) in experimental models of 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 250 patients with a hemispheric ischemic stroke of 7.8±4.5 hours’ duration. Early CT signs of cerebral infarction were evaluated on admission. The HT and infarct volume were analyzed from the CT performed on days 4 through 7. MMP-9 levels were determined by enzyme-linked immunosorbent assay in blood samples obtained on admission. Results— HT was observed in 38 patients (15.2%): 24 (63.2%) had a hemorrhagic infarction, and 14 (36.8%) had a parenchymal hematoma. A total of 108 patients (43%) received anticoagulants before the second CT scan. Systolic and diastolic blood pressures, body temperature, frequency of early CT signs of ischemia (92% versus 22%), and treatment with anticoagulants (79% versus 37%) were significantly higher in the group with HT ( P <0.001). Mean infarct volume was 126±60 cm ³ in the HT group and 90±68 cm ³ in the group without HT ( P =0.003). Median (quartiles) plasma MMP-9 concentrations were higher in the HT group (193 [163, 213] versus 62 [40, 93] ng/mL, P <0.001), even in the 24 patients seen within 3 hours of symptom onset ( P =0.014). MMP-9 levels ≥140 ng/mL had a positive and negative predictive value of HT of 61% and 97%, respectively. MMP-9 ≥140 ng/mL was associated with HT (odds ratio, 12; 95% confidence interval, 3 to 51; P <0.001) after adjustment for potential confounders and final infarct volume. Conclusions— High plasma MMP-9 concentration in the acute phase of a cerebral infarct is an independent biochemical predictor of HT in all stroke subtypes.