This study was undertaken to evaluate the safety and efficacy of switching patients treated with timolol maleate to timolol hemihydrate. In patients with ocular hypertension or chronic open-angle glaucoma treated with beta-blockers for at least three months, we prescribed timolol maleate solution 0.5% given twice daily for one month. We then switched each patient to timolol hemihydrate solution 0.5% (Betimol, Ciba Vision Ophthalmics) given twice daily for three months. This study found over the first three months in 30 completed subjects, using a worse eye analysis, that the intraocular pressure changed from 18.3+/-2.1 mm Hg on timolol maleate to 18.8+/-2.3 mm Hg on timolol hemihydrate (P=0.10) 12 hours after dosing. There was no difference in the overall incidence of unsolicited anterior segment side effects between timolol maleate (4 cases) versus timolol hemihydrate (3 cases) (P=0.69). One patient exited the study because of pain and burning in both eyes while on timolol maleate. No patient was discontinued due to loss of intraocular pressure control after switching to timolol hemihydrate. It was concluded that switching from timolol maleate to timolol hemihydrate is safe and effective in maintaining control of the intraocular pressure.