Mooren's ulcer is a chronic, painful corneal ulceration of unknown aetiology. Recent histological and immunological studies suggest an autoimmune basis. It is now becoming clear that the immune system plays an intricate role in maintaining homoeostasis in health and disease. Regulation of the immune response appears to involve a subset of peripheral blood T lymphocytes known as suppressor cells. A qualitative or quantitative deficiency of suppressor cells may therefore be responsible for chronic inflammation, autoimmune disease, and immunodeficiency states. To explain the reported immunological aberrations the number of suppressor T cells in addition to other immunological parameters were studied in a patient with bilateral Mooren's ulcers. A deficiency of suppressor T cells was found in the peripheral blood. This deficit in the immunoregulatory mechanism explains some of the immunological abnormalities reported in previous studies. Furthermore this study provides additional evidence for an autoimmune aetiology. In the light of these findings the possibility of a new line of treatment has been raised.