Parainfluenza virus uncommonly causes fatal giant cell pneumonia in immunocompromised infants and children. To our knowledge, this is the first adult case of parainfluenza virus pneumonia. A 77-year-old woman who was diagnosed as having small-cell carcinoma of the lung underwent chemotherapy. She died of lung edema. Analysis of her serum showed antibodies to parainfluenza virus types 2 and 3 at titers of 1:64 and 1:128, respectively. The postmortem examination revealed giant cell pneumonia, in which giant cells and detached alveolar lining cells had intracytoplasmic inclusions. On electron microscopic examination, the intracytoplasmic inclusions contained fuzzy-form nucleocapsids.