Data collected during the first five years after randomization in the Diabetic Retinopathy Study were analyzed to determine the effect of panretinal photocoagulation on intraocular pressure (IOP). At each follow-up visit, median IOP was identical for the treated and untreated eyes. Mean IOP rose slightly in each group. The proportion of untreated eyes with IOP above 30 mm Hg at two consecutive visits was twice that of the treated eyes (2% vs 1%). These data show that panretinal photocoagulation reduces the risk of subsequent intraocular hypertension, apparently by preventing the development of neovascular glaucoma.