This is a case report of a 47-year-old woman, who was admitted to hospital, due to severe cough and fever. A chest X-ray showed bilateral infiltrates, and sputum was found positive for Mycoplasma pneumoniae. She received treatment with intravenous clarithromycin but did not recover. After ten days macrolide-resistant M. pneumoniae was found in a throat swab. Moxifloxacin was administered, and she recovered quickly afterwards. Macrolide-resistant M. pneumoniae is an increasing problem and should be suspected, when there is no clinical effect on macrolides. Quinolones or tetracyclines may be used, when macrolide-resistant M. pneumoniae is detected.