We studied the bilateral intraocular pressure lowering effect of long-term unilateral timolol maleate 0.5% in 30 patients in whom early primary open-angle glaucoma had recently been diagnosed. A significant intraocular pressure reduction was found in the untreated fellow eyes (21.9 mm Hg from 26.4 mm Hg at baseline) (P less than .001), as well as in the treated eyes (21.0 mm Hg from 28.5 mm Hg at baseline) (P less than .001). Fourteen patients (47%) showed a significant bilateral intraocular pressure reduction (P less than .001), with little difference between the treated (8.2 mm Hg) and the untreated eyes (7.7 mm Hg) (P greater than .8).