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The purpose of this study was to describe a possible mechanism of failure of the Ahmed glaucoma valve (AGV; New World Medical, Rancho Cucamonga, CA).Two AGVS that failed to control intraocular pressure (IOP) were examined. In vivo examination consisted of unroofing the capsule surrounding the implant and injecting fluorescein-stained balanced salt solution into the tube. In one case the entire implant was removed and examined in vitro, and in the other case only the valve portion was removed.No flow in either case extended beyond the valve. In both cases, the membrane valve system was found to be fused and could not be pulled apart.Failure of the AGV may occur due to adhesion of the valve membranes.