In a controlled clinical study, 42 patients with dendritic keratitis were treated either with thermocautery plus human leukocyte interferon (HLI) or with minimal wiping debridement plus HLI. The results show that the efficiency with which primary debridement was carried out, determined the success of further therapy using interferon. We conclude that interferon works basically as a prophylactic agent and needs the support of further antiviral measures to be effective against dendritic keratitis.