Objective: Primary: to determine the loss of corneal endothelial cells after implantation of a Paul GDD with its tube either anterior or posterior of the iris. Secondary: to compare efficacy and safety of both procedures. Study design: Prospective, randomised, treatment controlled clinical trial. Study population: Pseudophakic patients with glaucoma who need a GDD. Intervention: Implantation of a Paul GDD with its tube anterior/posterior of the iris. Main study parameters/endpoints: Endothelial cell loss of the cornea. Nature and extent of the burden and risks associated with participation, benefit and group relatedness: At present potential benefits and drawbacks of positioning the Paul tube behind the iris are insufficiently known, successful positioning of the tube may require slightly more surgery time and the risk of hyphaema may be higher. It is expected, that in the long run damage to the corneal endothelium is less. Risks of study-related assessments are negligible, burden is low, extra time is about 35+15+35+35 minutes (total 2h).