AIM: To study the effect of an intravitreal injection of angiostatin on vascular leakage in the retina and iris of oxygen-induced retinopathy of prematurity (ROP). METHODS: Brown Norway rats at postnatal day 7 (P7) were exposed to hyperoxia (750mL/L O₂) for 5 days (P7-12) and then returned to normoxia to induce retinopathy. Angiostatin was reconstituted in sterile Phosphate Buffered Saline (PBS) and diluted to desired different concentrations. Angiostatin solution was injected into the vitreous of the right eye of the ROP rats at P14 and the age-matched normal rats through pars plana using a glass capillary, and the left eye received the same volume of sterile PBS as the control. Vascular permeability was quantified at 1, 2 and 3 days after the injection by measuring albumin leakage from blood vessels into the retina and iris using the Evans blue method and normalized by total protein concentrations. The expression of vascular endothelial growth factor (VEGF) in retina was evaluated using the Western Blot analysis and immunohis- tochemistry 24 hours following the injection. RESULTS: ROP rats showed significant increases of vascular permeability in the retina and iris (P <0.01). Angiostatin reduces vascular permeability in a dose-dependent manner in the retina of ROP rats. The reduction showed a time course trend. Angiostatin injection reduced retinal vascular permeability by approximately 1.5 and 2-fold at P15 (P <0.05) and P16 (P < 0.01), respectively. Angiostatin injection significantly reduced VEGF levels in the retina of ROP rats but did not affect retinal VEGF levels in normal rats. CONCLUSION: Angiostatin significantly decreases patholog- ical vascular permeability in the retina and iris of ROP rats but not in normal rats. Angiostatin down-regulates VEGF expression in retina of ROP rats. These results suggest that angiostatin may have a therapeutic potential in the treatment of ROP and other diseases with vascular leakage.