Sixty-four eyes undergoing Nd:YAG laser posterior capsulotomy were randomly assigned to pretreatment with 5 mg/ml (0.5%) timolol maleate in 32 eyes, and no drugs was given in the other 32 eyes as comparison. The result showed that the mean rise in intraocular pressure was significantly less in the timolol-pretreated group than that in the comparison group within 4 hours after laser procedure and the intraocular pressure was the lowest 2 hours after capsulotomy. The mean rise in intraocular pressure in pretreated group was less than that in the comparison group 24 or 48 hours after capsulotomy, but the difference was not significant. It is suggested that pretreatment with 5 mg/ml (0.5%) timolol could blunted the immediate post-capsulotomy pressure rise though delayed pressure increases could not be prevented. Therefore, it is recommended that timolol should be administrated before the YAG laser capsulotomy in order to reduce side effects caused by higher immediate post-capsulotomy pressure rise.