The cysteine protease interleukin-1 (IL-1) converting enzyme (ICE) cleaves inactive pro-IL-1β to active IL-1β. IL-1β is a pro-inflammatory cytokine that is implicated as an important pathophysiologic mediator of hypoxic-ischemic (HI) brain injury both in experimental models and in humans. Moreover, ICE-like proteases can also mediate apoptotic cell death. We hypothesized that neonatal mice with a homozygous deletion of ICE (ICE-KO)[Cell 80:401, 1995] would be resistant to brain injury after focal cerebral hypoxia-ischemia. P9-10 mice underwent right carotid ligation, followed by 70 min exposure to 10% O2. In 3 independent experiments, paired litters of ICE-KO (n=14) and wild-type (WT, n=16) mice derived from the same heterozygous progenitors underwent this lesioning procedure. Outcome was evaluated 5 days after lesioning; severity of injury was quantified by morphometric estimation of bilateral cortical, striatal and dorsal hippocampal volumes by image analysis. In a separate experiment, intra-HI surface temperature, and esophageal temperature over 2 h post-HI were measured in both groups. All WT mice had evidence of ipsilateral injury; neuropathologic findings in striatum, hippocampus and cortex ranged from regional atrophy to infarction. The incidence of cerebral infarction (defined as a visible cortical scar or cavitation detected on inspection of the brain) was significantly reduced in ICE-KO compared to WT mice (3/14 vs. 11/16 respectively, p<0.02, Fisher's exact test). In ICE-KOs, injury was markedly attenuated in all 3 regions mean ± SEM% reduction in right-sided regional volume compared to intact left-sided volume [100*(L-R)/L], WT vs. ICE-KO: striatum 50 ± 6 vs. 28 ± 4, p<0.002; cortex 44± 6 vs. 19 ± 5, p<0.005; hippocampus 48 ± 4 vs. 32±5, p<0.02. Intra- and post-HI temperatures did not differ between groups at any time. These results demonstrate that ICE activity contributes to the progression of neonatal brain injury in this model. Whether these deleterious effects are mediated by IL-1β production and/or by pro-apoptotic mechanisms is an important question for future studies.