OBJECTIVES:  The purpose of this study was to assess the incremental effect on intraocular pressure (IOP) reduction when changing from latanoprost-timolol fixed combination in the treatment of primary open-angle glaucoma (POAG).  METHODS:  Retrospective analysis was conducted of the Glasgow Royal Infirmary Glaucoma database.  The database was comprised of computerized medical records of all POAG patients treated at the Glasgow Royal Infirmary from 1981 to present.  Data elements recorded for each patient included demographics, diagnosis, and treatment history.  Treatment history included initial and subsequent medication regimens.  Patient IOP was measured before and after treatment change and mean change with 95% C.I. was calculated.  RESULTS:  Eighty-three cases of POAG were identified where treatment was changed from evening dosed latanoprost to latanoprost-timolol once daily to achieve or maintain lower target IOP.  The mean incremental reduction in IOP was 2.01 mmHg (95% C.I. 1.22-2.81).  CONCLUSIONS:  In POAG patients treated with latanoprost who require lower target pressures to control their disease, latanoprost-timolol fixed combination provides additional incremental IOP reduction.  Additional research should be conducted to further characterize and understand the importance of efficacy of combination therapy in the treatment of glaucoma.