Dry eye is a condition related to long-term topical eye therapy. We wish to evaluate the effectiveness and tolerability of preservative free prostaglandin drops versus benzalkonium chloride containing prostaglandin drops in the treatment of glaucoma. Patients undergoing prostaglandin monotherapy underwent a washout period of at least 1 month after which baseline measurements of dry eye severity were taken. Patients were randomised to receive either 0.0015% tafluprost drops or 0.005% latanoprost preserved with 0.02% benzalkonium chloride. Repeat measurements were taken after a 2-month interval. Thirty-five patients completed randomised treatment. No significant difference between groups was found in objective and subjective measurements of dry eye severity. No significant difference was found in measurement of treatment effectiveness. Preservative-free and benzalkonium chloride-containing drops were found to be equally effective in lowering IOP with no significant difference in either subjective or objective measurements of dry eye severity.