Large scale, multiclinic evaluations of vidarabine (Ara-A, Vira A, adenine arabinoside) for treating herpetic keratitis have been conducted as double-blind studies (169 patients) in comparison with IDU and open studies (146 patients). In the open studies, the disease in the majority of patients had been refractory to IDU. The effects of vidarabine and IDU were approximately the same in improvement of symptoms (lacrimination, photophobia, sensitivity) and percent of and time for corneal reepithelialization. With vidarabine, significantly more patients had improved distant visual acuity than did with IDU. In the open studies, vidarabine also was effective. Of 116 patients whose ulcers had not responded to IDU, 91 (78%) had reepithelialization within four weeks of treatment with vidarabine. On the basis of results from these studies, vidarabine appears to be a safe and effective drug for treating herpes simplex keratitis.