After anterior chamber paracentesis, patients responded with an effusion of protein into the anterior chamber. Pretreatment with aspirin significantly decreased this trauma-induced, probably prostaglandin-mediated, disruption of the blood-aqueous barrier. Protein levels in aqueous humor of human eyes were measured in samples taken initially and five minutes after anterior chamber tap. The protein rise of 53% in five minutes in control patients was limited to 7% in patients pretreated with systemically administered aspirin. Prostaglandin mechanisms may be important in human ocular trauma and prostaglandin synthetase inhibitors may control the reaction to surgical trauma.