The presence of a circadian variation of both intraocular pressure (IOP) and aqueous humor flow has been demonstrated in several studies. It must therefore be considered important to monitor IOP and evaluate the efficacy of ocular hypotensive drugs over the 24 hours of the day. The efficacy of latanoprost on IOP during both day and night has been evaluated and the most important results from four such studies are reviewed. The studies reviewed here clearly demonstrate that topical administration of latanoprost 0.005% once daily provided a steady reduction of the IOP during both day and night. Given as a single dose to healthy volunteers, latanoprost resulted in a sustained effect with a significant IOP reduction over 24 hours, and the reduction was still present, however less pronounced, even after 48 hours. Latanoprost administered once daily for 4 weeks to patients with glaucoma or ocular hypertension was more effective in reducing the IOP over 24 hours than timolol gel solution 0.5% once daily, timolol aqueous solution 0.5% twice daily, or dorzolamide 2% three times daily. Latanoprost applied once daily thus provided a better effect on the IOP together with a stable and sustained IOP reduction during both day and night.