Glaucoma is a serious disease that may cause a decrease in peripheral vision as well as blindness. Although the treatment of extreme levels of high intraocular pressure (p > 35 mm) reduces the risk of glaucoma it is not known if the treatment of mild and moderate intraocular pressure prevents the onset of glaucoma. A bibliographic search in Medline CD-ROM was performed from 1967 to 1995, as well as in SIGLE CD-ROM. The keywords used were 'glaucoma prevention' and 'glaucoma suspects'. After the search 18 papers which treated moderate and mild intraocular pressure were selected. Of all these papers, those that used the same methodology; randomization and timolol as treatment, were chosen. The possibility of publication bias was controlled by plotting effect size vs. number of papers, as well as effect size vs. year of publication. A meta-analysis using two methods, weighted zetas and adding zetas, was performed. The combined effect size was -6.25%, and the weighted effect size was -6.45%. We computed the pooled z by two methods: adding Zs and adding weighted Zs. The Z obtained by adding Zs has a value of 2.31120 p = 0.0177, one tail). Using the method of adding weighted Zs we obtained a value of 2.4201 (p = 0.0082, one tail). Our results show that the treatment with timolol of patients with mild and moderate intraocular pressure may help to prevent the onset of glaucoma.