Respiratory syncytial virus (RSV) is the most common cause of severe lower respiratory illness of infants and young children, causing 5%-40% of pneumonias and 60%-90% of bronchiolitis episodes. Peak incidence is at 2-3 months of age. About 30%-50% of children have been infected by 12 months of age and about 95% by age 5 years. However, no significant clinical immunity is produced; repeat infections may occur, and persons of any age may develop acute infection. The most common clinical illness is upper respiratory tract infection similar to the common cold. The virus is spread through airborne droplets. The incubation period is 2-8 days. Diagnosis can be made by culture, by tests for antigen, and by tests for antibody. The best specimen for culture is nasal washings; the next best is nasopharyngeal swab. In either case, the specimen should include cells from the posterior nose and pharyngeal epithelium, since they contain the virus. Swab specimens should be placed immediately into a transport medium, and any specimen should be placed into wet ice. However, culture is expensive, and usually must be sent to a reference laboratory with wet ice. Standard culture methods take 4-14 days; and shell-vial methods, 2-3 days. The virus survives only about 4 days at 4°C and dies quickly at ordinary freezer temperatures or at high temperatures. Culture (under optimal conditions) is still considered the gold standard for diagnosis. However, some investigators report less than optimum results (69%-83% sensitivity), especially with mailed-in specimens. Antibody detection methods include immunofluorescence and ELISA. Antibody detection methods have several drawbacks: the fact that sensitivity is often less than 50% in infants less than 3 months old; the need for acute and convalescent specimens unless sufficiently elevated IgM titers are present; and the necessity in most cases to send the serum specimens to a reference laboratory. Methods for antigen detection in patient specimens are also available,including fluorescent antibody and ELISA, with same-day results. Nasopharyngeal aspirates are reported to provide the best specimens. Compared to culture, sensitivity of these methods are about 80%-90% (range, 41%-99%). Antigen detection methods may be positive on some specimens that are negative by culture, especially with mailed-in specimens. Antigen detection is rapidly replacing culture and antibody detection for diagnosis of RSV infection. However, the sensitivity of different manufacturers’ kits may differ considerably.