Ocular hypertension (OHT) is defined as intraocular pressure (IOP) elevated above normal levels which occurs without detectable damage to the papilla or visual field. It is considered to be the most important risk factor for developing primary open-angle glaucoma (POAG). The "Ocular hypertension treatment study" (OHTS) addresses the issue of whether lowering IOC before damage to the papilla or visual field occurs can hinder their development and whether OHT can be treated categorically. This randomized, prospective, nonmasked and non-placebo-controlled, multicenter study documents the protective effect of prophylactic lowering of IOC and identifies and explains the risk factors for developing POAG. These include advancing age, reduced corneal thickness, increased pattern standard deviations in white-on-white perimetry, and elevated vertical or horizontal cup to disc ratio. Due to the large individual differences in risk distribution and the very high costs of treating OHT, the question of whether OHT should be treated on principle remains open in the OHTS. This decision must be made on an individual basis after each patient's risk has been assessed.