The aim of this randomized, prospective, masked clinical study has been to verify the influence of a non-steroidal anti-inflammatory drug ophthalmic solution on intraocular pressure reduction induced by 0.5% timolol and 0.005% latanoprost eyedrops in patients affected by primary open-angle glaucoma. Thirty-two glaucomatous patients, compensated with 0.5% timolol, were randomized into two study groups (A and B). Timolol was continued for the first 2 weeks in all subjects. On the 15th day, in both groups timolol was replaced by latanoprost, and this regimen lasted up to the end of the follow-up (8 weeks). At the beginning of the 2nd week of the study, group A additionally started a 5-week therapy with topical 0.1% diclofenac; during the same period, group B received placebo eyedrops with identical modalities. Intraocular pressure was recorded at 7-day intervals during the first 7 weeks and at the 10th week. Non-steroidal anti-inflammatory drug and placebo did not modify the effect of timolol on intraocular pressure. In both groups, latanoprost induced a significant decrease in intraocular pressure. Diclofenac-treated patients exhibited a marked fall in intraocular pressure (p<0.01), whereas in placebo-treated patients, this diminution was less noticeable (p<0.05). After diclofenac withdrawal, in group A intraocular pressure significantly increased (p<0.01), remaining approximately at the same level up to the end of the study. In group B, at the same checks no significant variations in intraocular pressure occurred. In primary open-angle glaucoma patients, diclofenac significantly enhances the hypotensive effect of latanoprost, without influence on timolol efficacy. Because non-steroidal anti-inflammatory drugs are widely employed in medical practice, supplementary ophthalmologic checks should be scheduled during the co-administration of these compounds and prostaglandin analogues.