The effectiveness of dorzolamide 2%, a topical carbonic anhydrase inhibitor, administered three times daily (TID) concomitantly with timolol 0.5% given twice daily (BID) was evaluated in one arm of a large, 1-year, multicenter efficacy and safety trial. That study compared dorzolamide 2% TID with betaxolol 0.5% BID and timolol 0.5% BID as monotherapy in patients with open-angle glaucoma or ocular hypertension. Ninety-seven patients with uncontrolled intraocular pressure (IOP) (ie, greater than 21 mm Hg) or with a reduction in IOP of less than 15% from baseline during monotherapy with dorzolamide required 0.5% timolol BID as adjunctive therapy; 95 patients were evaluated for efficacy. IOP was measured 1 week after starting adjunctive therapy and at all subsequent study visits. Data were evaluated as change in IOP from original baseline and change in IOP from end of monotherapy treatment. A clinically meaningful additive effect on IOP was observed at all time points. After 1 week of adjunctive therapy, the mean percent reduction from baseline in peak and afternoon trough IOPs was 34% and 28%, respectively. These results demonstrate that dorzolamide and timolol have an additive effect in lowering IOP. This combination was generally well tolerated by the patients in this study.