Men and nonpregnant women between the ages of 40 and 80 with IOP greater than or equal to 24 mm Hg but less than or equal to 32 mm Hg in at least one eye and IOP greater than or equal to 21 but less than or equal to 32 mm Hg in the fellow eye, as well as normal visual fields and optic discs are eligible for the trial. Patients presenting with best-corrected visual acuity worse than 20/40 in either eye, previous intraocular surgery, a life-threatening or debilitating disease, secondary causes of elevated IOP, angle-closure glaucoma or anatomically narrow angles, other diseases that can cause visual field loss, background diabetic retinopathy, optic disc abnormalities that can produce visual field loss or obscure the interpretation of the optic disc, or unwillingness to undergo random assignment are excluded from the trial.