Sixteen patients (21 eyes) with severe, progresssive Mooren's ulcer, unresponsive to medical or surgical therapy, were treated with 0.5% cycllosporin A (CsA) eyedrops. Only 18 eyes were included in this study. Fifteen eyes showed marked effects within the first 12 days. Eleven eyes completely healed, on the average, in 1.3 months without side-effects. Follow-up for 24-31 months showed no recurrence. The whole-blood level of CsA determined randomly by high-pressure liquid chromatography in 7 patients was 28.95-mu-g/L on the average. It is concluded that autoimmunological mechanism plays an important part in pathogenesis of Mooren's ulcer, and the efficacy of topical CsA is attributable to its effect of local depression of ocular immunopathological reactions.