A 42-year-old woman was referred to our hospital with a diagnosis of influenza A and pneumonia out of the influenza season. Chest CT findings resembled interstitial pneumonia, but we initiated anti viral agents and antibiotics. Flexible fiberoptic bronchoscopy was performed on hospital day 3. Based on the results of an RT-PCR analysis of broncho-alveolar lavage, this patient was diagnosed as having influenza viral pneumonia. In the influenza season, we can easily suspect influenza as a differential diagnosis, even if the patient's chest CT findings resemble interstitial pneumonia. Out of the influenza season, clinicians should take into consideration influenza viral pneumonia as a differential diagnosis.