Parainfluenza type 3 (PIV 3) is a well-recognized cause of respiratory illness after stem cell transplantation (SCT), with an estimated incidence of 2-7% and a high mortality rate associated with lower respiratory tract infection (LRTI). A 12-month retrospective study was undertaken in which 23 positive cases of PIV 3 occurred in SCT recipients. The frequency of infection was 36.1% in matched unrelated donor SCT recipients, 23.8% in sibling allogeneic SCT recipients and 2.3% in autologous transplant recipients. Seventeen cases were outpatient or community acquired despite standard infection control measures. Eleven patients only developed upper respiratory tract symptoms. LRTI symptoms developed in 12 patients, of whom eight had a new infiltrate on chest X-ray. Overall mortality at 30 days from PIV 3 diagnosis was 4% (one patient). Four patients died within 100 days of PIV 3 diagnosis, but PIV 3 was not believed to be the primary cause of death in any of these patients. Early ribavirin was used in eight patients and only one patient who received ribavirin died. These results suggest a higher prevalence of PIV 3 but a lower mortality than documented previously, particularly in allogeneic transplant recipients. The authors propose that the high prevalence reflects the unit's policy of active surveillance for respiratory viruses and the difficulty in preventing transmission of PIV 3, especially in the outpatient setting during an outbreak period. Ribavirin treatment may improve outcome in patients with LRTI but is not required in all patients with PIV 3.