OBJECTIVE Immunological factors play an important role in the development of herpetic stromal keratitis. T lymphocytes are the principal cells involved in this immunologic reaction. We therefore investigated the efficacy and safety of topical cyclosporin A 2 % solution in herpetic stromal keratitis. METHODS Ten patients with herpetic stromal keratitis received topical cyclosporin A 2% 4 times daily for 2 months. Acyclovir 3% ointment 5 times daily and cycloplegic eyedrops 2 times daily were used in conjunction with cyclosporin A solution in the 1st month of treatment. Topical steroids were not used. The patients were followed for 6-9 months. Treatment efficacy was evaluated according to two parameters: resolution of stromal infiltrates and neovascularization and increase in visual acuity by two or more Snellen lines. Serum cyclosporin A levels were measured with high-performance liquid chromatography. RESULTS In all the patients, the stromal infiltration resolved completely after 2 months of treatment. Visual acuity increased by two or more Snellen lines in 8 out of 10 patients. In the remaining 2 cases, stromal scarring that was present before cyclosporin A treatment prevented vision increase. There were no serious complications from the cyclosporin treatment but 4 cases complained of severe burning upon instillation of the drug. This complication was temporary and did not result in discontinuance of the drug. Serum levels of the drug were always below toxicity levels. CONCLUSIONS While this is a noncontrolled study, our results show that topical cyclosporin can be a valuable adjunct to acyclovir treatment in herpetic stromal keratitis. Use of topical cyclosporin in dendritic herpetic ulcers and necrotic stromal keratitis has not been investigated in this study.