Background. Although imaging patterns of infectious pneumonia vary widely, unilateral lobar pneumonia is rare in patients with primary viral pneumonia. Case. A 41-year-old woman presented with back pain, cough, and a fever. Computed tomography of the chest showed lobar pneumonia with bronchiolar translucency in the right upper and middle lobes, and she was admitted to our hospital. Bronchoscopy was performed because the patient did not respond to broad-spectrum antimicrobial therapy. Although no significant pathogens were cultured from the bronchoalveolar lavage fluid, it was positive for adenovirus type 3 by polymerase chain reaction. Because the anti-adenovirus antibody titer was significantly elevated in paired sera, a diagnosis of primary adenovirus pneumonia was made. Conclusion. When antimicrobial agents are ineffective in lobar pneumonia, the possibility of adenoviral pneumonia should be kept in mind.