We report a case of cystic fibrosis in a 19-year-old woman who suffered from frequent exacerbations of lower respiratory infection due to multidrug-resistant Pseudomonas aeruginosa and who was successfully treated with parenteral colistin. Multidrug-resistant Pseudomonas aeruginosa isolated from sputum had become resistant to all parenteral antibiotics commercially available in Japan. She did not show clinical improvement despite treatment with several different combinations of available antibiotics. We therefore obtained parenteral colistin from a pharmacy outside Japan. She responded well to parenteral colistin without apparent side effects such as serious nephrotoxicity or neurotoxicity. Colistin is therefore an important alternative antibiotic for treating multidrug-resistant Pseudomonas aeruginosa and its use should be considered in severe infection. We hope that parenteral colistin will become available in Japan in the near future.