The histological findings and clinical course of primary seasonal influenza viral pneumonia have not been fully elucidated. We herein report the case of a 65-year-old man with primary seasonal influenza viral pneumonia. The patient presented with fever, myalgia, general fatigue and dyspnea of seven days duration. Chest computed tomography showed bilateral ground-glass opacity and consolidation. A rapid influenza virus antigen test was positive for influenza A virus. We diagnosed him as having community-acquired influenza pneumonia and started therapy with antibiotics plus oseltamivir; however, his symptoms, respiratory condition and radiological findings deteriorated. Polymerase chain reaction following bronchoscopy showed the bronchoalveolar lavage fluid to be positive for the influenza A virus. A right lower lobe transbronchial lung biopsy revealed type II pneumocyte metaplasia, acute and chronic interstitial infiltrates and alveolar organization without hyaline membranes or fibrin thrombi in the vascular lumen. Treatment with prednisolone at a dose of 60 mg/day (1 mg/kg) resulted in an improvement. The patient was discharged on hospital day 15. Two weeks after admission, the serum antibody titer for influenza A (H3N2) had increased significantly. No other pathogens were found either serologically or in the respiratory samples, and we diagnosed the patient to therefore have primary influenza viral pneumonia.