An aqueous-drainage silicone device was implanted in a 57-year-old diabetic patient with neovascular glaucoma after failure of conventional therapy. Postoperative intraocular pressure (IOP) was well controlled, ranging between 10 and 20 mm Hg without the use of supplemental agents. Despite the normal IOP, the eye became painful and amaurotic 11 months after implantation due to progression of the retinopathy accompanied by the development of severe tractional retinal detachment. Enucleation was performed and the eye underwent histopathological examination. The filtering bleb around the silicone plate of the implant on the equatorial sclera was lined with a thick layer of collagenous connective tissue. Numerous microcystic spaces could be seen in the bleb wall by light microscopy. Electron microscopy confirmed this finding; in addition, only a meshwork of collagen fibre-like bundles was visible at some areas of the innermost bleb wall. Control of IOP in spite of bleb fibrosis in this particular case may have been due to a loss of matrix components and to the formation of microcystic spaces in the bleb wall, which seem likely to be channels for the passage of aqueous humor.