A 78-year-old woman complained of ocular pain in the right eye. Thirty-three months before, the patient had a pterygium excised and mitomycin C, 0.04%, was instilled 3 times a day postoperatively for 10 days in the right eye. The same surgery was performed and the same drug was used for a course of 5 days in the left eye. Corneoscleral ulceration and corneal performation occurred in the right eye. The lesions were successfully treated with a lamellar patch graft with reserved corneosclera. We believe that it may not be safe to instill 0.04% mitomycin C 3 times a day postoperatively for 10 days.