Feline infectious peritonitis is a fatal viral disease which is currently incurable by drugs. Also, there are no effective vaccines. Cyclosporin A (CsA) was recently shown in vitro to exert potent antiviral activities on several virus-host systems, including various coronaviruses. However, whether CsA has clinically relevant activity in vivo resistant to various coronaviruses, including the severe acute respiratory syndrome and Middle East respiratory syndrome coronaviruses, remains unknown. Herein, the authors report that treatment with CsA caused a sustained reduction in pleural fluid volume and viral copy number in a cat diagnosed with effusive feline infectious peritonitis.