A follow-up study was performed on 54 eyes with chronic open-angle glaucoma treated with timolol maleate. It was carried out as an open clinical trial with a selected group of eyes. During 2 years of application the effect of timolol under normal conditions as well as under stress tests (darkness test, water-, coffee- or tea-drinking test, early morning measurements) was observed. Until now 40% of patients were regulated with 0.25% timolol twice daily. Another 50% obtained good pressure values under 0.5% timolol therapy and the rest with 0.5% timolol and 2% pilocarpine twice daily. Early morning measurement was shown to be the most important indicator of intraocular pressure regulation by β-blocking agents. While timolol is usually very effective in reducing the intraocular pressure during daytime, several patients showed unattended high morning pressure values. Up to now, no severe side-effects of timolol therapy have been observed.