We studied the ability of topical clonidine, an alpha-agonist, to suppress the acute rise in intraocular pressure (IOP) following neodymium-YAG posterior capsulotomy (YPC). In a randomized controlled trial 63 eyes were pretreated with one drop of either 0.25% clonidine or saline 1 h before performing YPC and immediately following the procedure. The greatest IOP rise in the saline treated eyes occurred in the second hour after YPC, when the mean (+/- SD) IOP rose from a baseline pressure of 12.7 +/- 3.2 to 18.7 +/- 10.7 mmHg. In clonidine treated eyes, the IOP fell from a mean of 11.9 +/- 3.4 to 9 +/- 3.3 mmHg 2 h postoperatively. In the saline treated group 9 eyes (27%) developed an IOP rise greater than 10 mmHg. Clonidine proved to be highly effective in preventing the rise in IOP following YPC.