Objective: To determine whether the Paul tube induces less damage to the corneal endothelium than the Baerveldt GDD. Study design: Randomized clinical trial. Study population: Phakic patients scheduled for surgical GDD implantation. Intervention: Either a Baerveldt or a Paul GDD implant. Main study parameters/endpoints: Endothelial cell density and tube position at 24 months. Nature and extent of the burden and risks associated with participation, benefit and group relatedness: The Paul GDD may have a less harmful effect on corneal endothelium. Otherwise, both GDDs will probably have a similar risk/benefit profile. The risks of study-related assessments are negligible, burden is low, extra time is about 5 x 1.5 h (total 7.5 h) in two years.