Parainfluenza virus type 3(PIV-3) commonly causes respiratory tract infections in hematopoietic stem cell transplant (HSCT) patients. The majority of PIV-3 infections develop in patients who have undergone stem cell transplantation from unrelated donors. From these patients, bronchoalveolar lavage (BAL) fluid and/or lung biopsies are often collected and sent for evaluation of infectious processes. However, cytologic findings associated with a PIV-3 infection in BAL fluid have not been reported in the literature. We describe BAL cytology and lung biopsy findings in a patient who received an HSCT from a related donor and subsequently developed a PIV-3 infection. This patient was noted to have scattered reticular-nodular opacities in both lungs on computed tomogram scan and underwent transbronchial biopsy and BAL of the left lower lobe. Examination of the BAL fluid revealed scattered multinucleated giant cells intermixed with inflammatory cells. The lung biopsy showed organizing pneumonia associated with several multinucleated respiratory epithelial cells containing rare intracytoplasmic inclusions. Gram, periodic acid Schiff, Gomori methenamine silver, and acid fast stains on the biopsy specimen failed to reveal microorganisms. A sample of the BAL fluid sent for respiratory viral culture grew PIV-3. These findings suggest that the presence of giant cells in transplant patients with organizing pneumonia should raise suspicion of a PIV-3 infection. Diagn. Cytopathol. 2014;42:521–524. © 2013 Wiley Periodicals, Inc.