Inhibition of matrix metalloproteinase-9 (MMP-9) protects the adult brain after cerebral ischemia. However, the role of MMP-9 in the immature brain after hypoxia–ischemia (HI) is unknown. We exposed MMP-9^(−/−)[MMP-9 knock-out (KO)] and wild-type (WT) mice to HI on postnatal day 9. HI was induced by unilateral ligation of the left carotid artery followed by hypoxia (10% O₂; 36°C). Gelatin zymography showed that MMP-9 activity was transiently increased at 24 h after HI in the ipsilateral hemisphere and MMP-9-positive cells were colocalized with activated microglia. Seven days after 50 min of HI, cerebral tissue volume loss was reduced in MMP-9 KO (21.8 ± 1.7 mm³;n= 22) compared with WT (32.3 ± 2.1 mm³;n= 22;p< 0.001) pups, and loss of white-matter components was reduced in MMP-9 KO compared with WT pups (neurofilament: WT, 50.9 ± 5.4%; KO, 18.4 ± 3.1%;p< 0.0001; myelin basic protein: WT, 57.5 ± 5.8%; KO, 23.2 ± 3.5%;p= 0.0001). The neuropathological changes were associated with a delayed and diminished leakage of the blood–brain barrier (BBB) and a decrease in inflammation in MMP-9-deficient animals. In contrast, the neuroprotective effects after HI in MMP-9-deficient animals were not linked to either caspase-dependent (caspase-3 and cytochromec) or caspase-independent (apoptosis-inducing factor) processes. This study demonstrates that excessive activation of MMP-9 is deleterious to the immature brain, which is associated with the degree of BBB leakage and inflammation. In contrast, apoptosis does not appear to be a major contributing factor.