A double-masked comparative clinical study of UF-021 (Rescula), a unique prostaglandin-related compound, was carried out at 18 centers with timolol maleate 0.5% ophthalmic solution as an active reference drug. After a wash-out period, UF-021 (0.12%) or timolol (0.5%) was given topically twice a day for 12 weeks to 158 patients with primary open-angle glaucoma or ocular hypertension, in a randomized double-masked manner. Both groups showed a significant reduction in intraocular pressure from the second week to the end of the study. In overall improvement rating, 91.4% of the cases (64/70) in the UF-021 group and 88.3% (68/77) in the timolol group were judged to be "Extremely improved" or "Improved". Five and 4 cases reported having side effects in the UF-021 and timolol groups, respectively, but none of the cases required any change or discontinuation of treatment. While UF-021 did not affect blood pressure, both systolic and diastolic blood pressures in the timolol group were significantly decreased. These results suggest that UF-021 (0.12%) has the potential to lower intraocular pressure equivalent to timolol (0.5%), while having no effect on cardiovascular functions.