A 75-year-old man first developed dyspnea and low-grade fever in late March. A chest X-ray film showed infiltration in the right lower lung field and blood gas analysis revealed severe hypoxemia. Accordingly, he was diagnosed as having pneumonia and was admitted to our hospital on March 11, 2003. Mechanical ventilation for progressive respiratory failure was started immediately after admission, and he was treated with antibiotics. Chlamydia pneumoniae pneumonia was diagnosed due to an increase of the Chlamydia pneumoniae antibody titer. He had prolonged respiratory failure despite antibiotic therapy. Therefore, steroid therapy was started on day 15 for respiratory failure. At 21 days after admission, the infiltration was found to be decreased on chest X-ray films and improvement of hypoxemia allowed extubation. In conclusion, when severe community-acquired pneumonia occurs in elderly patients, we should remember the possibility of atypical pneumonia such as that due to Chlamydia pneumoniae infection.