Thirty-five patients with long-standing deep stromal herpes simplex keratitis were treated with 3% acyclovir ointment and corticosteroids. All patients were cured in 2 to 4 weeks, independent of both the duration of the keratitis before acyclovir therapy and the previous antiviral medication. Corticosteroids in tapering dosages had to be continued for many months after healing to prevent recurrence of stromal corneal oedema; recurrences could not be prevented by acyclovir therapy. No serious side-effects of acyclovir were noted.