We compared the ocular hypotensive effect for 24 hours and the tolerability of latanoprost stored at 4 degrees C and 30 degrees C. Seventeen healthy volunteers were included in this crossover trial. Latanoprost 0.005% (Xalatan) was stored at 4 degrees C or 30 degrees C for 4 weeks in the dark. The subjects enrolled to the study were randomly assigned to receive either latanoprost stored at 4 degrees C or that stored at 30 degrees C. The eye drop was applied to the right eye of each subject for 3 days. The left eye served as a control without administration. Slit-lamp biomicroscopy and circadian intra ocular pressure (IOP) curve was performed at Day 3, every 3 hours from 6 pm. This procedure was repeated 7 days after changing the drug from 4 degrees C to 30 degrees C or vice versa, and application to the left eye for 3 days. Eyes treated with latanoprost, stored both at 4 degrees C and 30 degrees C, achieved statistically significantly lower mean IOPs than untreated eyes at all time points, except at 21 hours treated by the drug stored at 30 degrees C. We subtracted the IOP of eyes receiving latanoprost from the IOP of untreated eyes for each time point to evaluate the efficacy of the eye drops (delta IOP). There were no statistically significant differences between the delta IOPs with the drug stored at 4 degrees C and 30 degrees C. During the study, no subject developed a serious adverse event. These results suggest that latanoprost stored at 30 degrees C for 4 weeks after opening the bottle remains as effective and safe as latanoprost stored under cold conditions.