We reviewed the frequency and clinical course of parainfluenza virus (PIV) infections in 1,173 adult bone marrow transplant (BMT) recipients cared for at The University of Texas M.D. Anderson Cancer Center (Houston). Between January 1991 and September 1994, PIV was isolated from the respiratory secretions of 61 (5.2%) of these patients. Thirty-four (56%) of the 61 patients had uncomplicated upper respiratory tract illnesses and survived. The remaining 27 patients (44%) developed pneumonia, and the associated mortality was 37% (10 of 27 patients). Twenty-three (85%) of the patients with pneumonia had had preceding upper respiratory illnesses. Of the 10 patients who died, nine died within 100 days after transplantation. Histopathologic examination of lung tissue from seven patients revealed intracytoplasmic viral inclusions in six, a finding consistent with invasive PIV pneumonia, and viral changes in the seventh patient. Seven of the 10 patients who died had other serious concurrent infections. Of 42 patients who developed PIV infection early after transplantation (i.e., < 100 days), the frequency of pneumonia was higher among the 18 allogeneic BMT recipients (61%) than among the 24 autologous BMT recipients (42%), and the associated mortality was also higher (55% vs. 30%, respectively). PIVs are an important cause of life-threatening pneumonia in adult BMT recipients, particularly patients who have recently undergone allogeneic bone marrow transplantation.