Respiratory syncytial virus (RSV) and parainfluenza virus (PIV) are common causes of respiratory infections in immunocompetent children under the age of 6 years. These viruses belong to a family of enveloped single-stranded RNA viruses, the paramyxoviruses. The clinical manifestations in the normal host range from mild illness to severe croup, bronchiolitis, and pneumonia. After the age of 6 years, reinfections occur, but are characterized by diminishing frequency and severity. In contrast to the normal adult host, severe lower respiratory tract infection can occur in immunocompromised adults as well as children, with significant morbidity and mortality. Similar to the normal host, infection with RSV occurs in epidemics in the winter and spring, while PIV occurs throughout the year. Immunocompromised hosts often have upper respiratory tract symptoms similar to those experienced by normal hosts, as well as a higher incidence of lower respiratory tract symptoms and sinusitis. Lower respiratory tract infection can lead to respiratory failure and death, especially in bone marrow transplant recipients. The diagnosis of RSV and PIV depends on the analysis of specimens obtained from the respiratory tract. Rapid diagnostic tests are readily available for RSV and are less widely used for some of the PIV serotypes. Primary cultures are used for both viruses, but take several days to yield a positive result. Ribavirin, a broad-spectrum antiviral agent, is effective against RSV and PIV in vitro. Clinical trials have shown ribavirin to be of benefit in treating infants infected with RSV. However, clinical trials in immunocompromised patients infected with RSV or PIV have not been carried out. Since infection with RSV and PIV can be severe and life-threatening and treatment with ribavirin is relatively benign, it seems warranted to treat immunocompromised patients infected with RSV or PIV with ribavirin until otherwise proven unwarranted.