BACKGROUND: Acyclovir (ACV) ophthalmic ointment is effective in the treatment of herpetic keratitis. However, when applied, the ointment has an unpleasant feeling and some cases are resistant to ACV. A new antiviral compound, carbocyclic oxetanocin G (C.OXT-G) has potent anti-herpes simplex virus activity and high water solubility, so the clinical effect of C.OXT-G eyedrops on ulcerative herpetic keratitis was evaluated. METHODS: Studies were conducted on the corneal ulcers in 37 eyes of 27 patients. Patients with typical dendritic or geographic corneal ulcers were treated with 0.1% C.OXT-G eyedrops, applied five times a day, together with eyedrops of an antibiotic applied four times a day. The eyes were examined at least twice a week until the ulcers healed, and thereafter at intervals for up to 3 months. RESULTS: All of the ulcers healed, their average healing time being 4.9 (SD 2.2) (range 2 to 9) days. The ulcers in 20 of the 37 eyes were induced by the use of corticosteroid or immunosuppressive drugs, and their average healing time was 4.8 (2.3) days. No adverse drug reactions were seen during the observation period in this trial. CONCLUSION: Eyedrops containing 0.1% C.OXT-G are excellent and safe for treatment of herpes simplex corneal ulcers in humans.