Purpose To compare the nocturnal effects of once-daily timolol and latanopros t on intraocular pressure (IOP) in patients with ocular hypertension or early gl aucomatous changes. Design Prospective, open label, experimental study with cro ssover design. Methods Eighteen patients with ocular hypertension or early glauc omatous changes (aged 41 to 79 years) each received topical treatments with timo lol (0.5%Timoptic XE), latanoprost (0.005%Xalatan), and no IOP lowering medi cation, for at least 4 weeks. Timolol was given once in the morning upon awakeni ng and latanoprost once in the evening at bedtime. At the end of each treatment period, the patient was housed in a sleep laboratory for 24 hours and IOP was me asured every 2 hours using a pneumatonometer. Measurements were taken sitting an d supine during the 16 hour diurnal/wake period and only supine during the 8 h our nocturnal/sleep period. Mean diurnal and nocturnal IOP levels were compared among the treatments with timolol, latanoprost, and no medication. Results In th e diurnal period, the mean IOP under the timolol or the latanoprost treatment wa s significantly less than the mean IOP under no medication in both the sitting a nd the supine positions. There was no statistical difference between the timolol and latanoprost treatments. In the nocturnal period, supine IOP with timolol tr eatment was not different from the supine IOP with no medication but was signifi cantly higher than supine IOP with the latanoprost treatment. Conclusion Althoug h both once daily timolol and latanoprost were effective in lowering IOP during the diurnal period, only latanoprost reduced IOP during the nocturnal period. Purpose To compare the nocturnal effects of once-daily timolol and latanopros t on intraocular pressure (IOP) in patients with ocular hypertension or early gl aucomatous changes. Design Prospective, open label, experimental study with cro ssover design. Methods Eighteen patients with ocular hypertension or early glauc omatous changes (aged 41 to 79 years) each received topical treatments with timo lol (0.5% Timoptic XE), latanoprost (0.005% Xalatan), and no IOP means mediation, for at least 4 weeks. Timolol was given once in the morning upon awakeni ng and latanoprost once in the evening at bedtime. At the end of each treatment period, the patient was housed in a sleep laboratory for 24 hours and IOP was me as used every 2 hours using a pneumatonometer. Measurements were taken sitting an d supine during the 16 hour diurnal / wake period and only supine during the 8 h our nocturnal / sleep period. Mean diurnal and nocturnal IOP levels were compared among the treatments with timolol, latan oprost, and no treatment. Results In th e diurnal period, the mean IOP under the timolol or the latanoprost treatment wa s significantly less than the mean IOP under no medication between both sitting a nd the supine positions. There was no statistical difference between the timolol and latanoprost treatments. In the nocturnal period, supine IOP with timolol tr eatment was not different from the supine IOP with no medication but was signifi cantly higher than supine IOP with the latanoprost treatment. Conclusion Althoug h both once daily timolol and latanoprost were effective in lowering IOP during the diurnal period, only latanoprost reduced IOP during the nocturnal period.