Defects in apoptosis have been noted in signal transducer and activator of transcription (Stat) 1-null cells in vitro. The purpose of this study was to analyse the keratocyte apoptosis response that occurs in vivo in response to corneal epithelial injury in Stat 1null compared with control mice and to determine whether Stat 1null corneal fibroblasts have a defective response to death receptor activation in vitro. Corneal epithelial scrape injuries were performed in Stat 1-null and wild-type mice. Keratocyte apoptosis was monitored with the quantitative TUNEL assay and confirmed using transmission electron microscopy. Corneal fibroblast apoptosis in response to tumor necrosis factor (TNF) alpha, with and without inhibitors of nuclear factor kappa B (NF-kappaB) activation, was monitored using DNA laddering and the methylene blue assay. Significantly less keratocyte apoptosis was noted in Stat 1-null mice compared with wild-type controls. TNF alpha-induced apoptosis only occurred in wild-type mice in the presence of inhibitors of NF-kappaB activation. Corneal fibroblast TNF alpha-induced apoptosis was defective in Stat 1null corneal fibroblasts whether NF-kappaB activation was blocked or not. Stat 1 has an important role in the keratocyte apoptosis that occurs in response to corneal epithelial injury. Previous studies suggest that the defect is due to a lack of constitutive expression of caspases. This study demonstrates that this defect in apoptosis in Stat 1-null mice is present in vivo in Stat 1-null mice and suggests that Stat 1 could be a therapeutic target for transient inhibition of keratocyte apoptosis to modulate corneal wound healing.