We studied apraclonidine hydrochloride (aplonidine hydrochloride or ALO 2145), an alpha-agonist, for its effect on the intraocular pressure (IOP) rise following neodymium-YAG posterior capsulotomy (YPC). In a prospective multicentered double-masked study, 63 eyes were pretreated with one drop of either 1% apraclonidine hydrochloride or placebo one hour before performing YPC and again following the laser treatment. The greatest IOP rise in the placebo-treated eyes occurred in the third hour after YPC, when the mean (+/- SD) IOP rose from a baseline pressure of 16.4 +/- 3.7 to 20.8 +/- 6.8 mm Hg. In apraclonidine-treated eyes, the IOP fell from a mean of 15.6 +/- 3.8 to 12.8 +/- 6.0 mm Hg three hours postoperatively. There were five times as many eyes that had an IOP rise greater than 10 mm Hg in the placebo-treated group compared with those treated with apraclonidine. Apraclonidine proved to be highly effective in preventing the rise in IOP following YPC.