16 patients (21 eyes) with severe progressive Mooren's ulcer unresponsive to medical or surgical therapy were treated with 0.5% cyclosporin A (CsA) eyedrops. 15 eyes showed remarkable amelioration within 12 days, and 11 eyes completely healed without side effects in an average 1.3 months. The total effective rate was 83.3%, with no recurrence in a follow-up of 12-19 months. Whole-blood levels of CsA were determined by high pressure liquid chromatography to average 28.95 ng/ml in 7 random patients. Autoimmunity was believed to be an important factor in the pathogenesis of Mooren's ulcer, and the efficacy of topical CsA was attributable to the local depression of ocular immunopathological reactions.