SUMMARY A randomised double blind study was performed on 59 patients with dendritic herpetic keratitis treated with 3% acyclovir or 2% trifluorothymidine ointment. Both drugs were highly effective and gave 87% (ACV) and 82% (T1F') success rates. Punctate keratopathy occurred in 70% of the patients, but serious side effects were not observed.Since the introduction of idoxuridine (IDU) as an antiviral drug for the treatment of herpetic keratitis more potent drugs have become available. Arabinoside adenine and trifluorothymidine (TFT) are both effective and relatively nontoxic alternatives widely used in the management of herpetic disease.' Acyclovir (ACV) (9-(2-hydroxyethoxymethyl) guanine)2 and (E)-5(2-bromovinyl)-2' deoxyuridine),3 recently developed antiviral compounds with potent inhibitory activity against herpes simplex DNA polymerase, do not inhibit normal human cell DNA polymerase.For a comparative study we selected 2 of the antiviral drugs which we considered to be the most potent -acyclovir and trifluorothymidine.45 Material and methods Fifty-nine patients with dendritic keratitis were studied. Patients with stromal keratitis and patients treated by antiviral agents and/or corticosteroids prior to referral were excluded. In a randomised double-blind study we used 3% acyclovir and 2% trifluorothymidine ointment, both specially prepared for this trial and packed in identical tubes obtained through the kind co-operation of Wellcome Laboratories (England) and Dr Gerhard Mann, Hamburg (FRG), respectively. The ointment was prescribed 5 times daily. Cycloplegics were given for intraocular irritation.After healing was complete the treatment was continued for 7 days. It was discontinued if after 7 days no therapeutic effect was observed or if after 14 days the keratitis had not healed completely.