The definitive diagnosis and determination of recurrence of herpes simplex keratitis are still difficult in clinical ophthalmology. At present, isolation of virus by tissue culture is perhaps the best method for establishing a specific aetiological diagnosis of viral infection. But due to its complicated and time-consuming procedures, the application of tissue culture for virus isolation in clinical work is still limited. In situ DNA hybridization is a specific and quick technique for directly detecting genetic materials, DNA and RNA, of viruses. In this study, this technique was used to identify herpes simplex virus type 1 from a patient's cornea suffered from recurrent herpetic keratitis. The technique offers a convenient and specific method for clinicians to make a definitive diagnosis and differential diagnosis of viral infectious diseases. The advantages and disadvantages of other different methods available for viral diagnosis, such as light and electron microscopy and immunohistochemistry were discussed with an emphasis on in situ DNA hybridization.