This prospective clinical study comparing laser trabeculoplasty, medical therapy and surgery as the primary therapy for 168 patients with untreated chronic simple glaucoma, showed that the surgical group resulted in the lowest mean intraocular pressures, and was most effective in controlling the diurnal swing of intraocular pressures. Laser trabeculoplasty resulted in a lesser degree of pressure reduction but these patients were more frequently subject to high pressure spikes. Medical therapy was the least effective in lowering pressure.