We studied the frequency and severity of respiratory syncytial virus infections acquired nosocomially on an infants' ward during a community outbreak. Every three or four days all infants and staff were examined, and specimens were obtained for viral isolation. During two months, 14 of 44 contact infants acquired the virus. All were ill, and four had pneumonia. Infected infants had a significantly longer mean hospital stay (21.5 days) than uninfected ones (9.2 days, P less than 0.001). Risk of nosocomial infection could not be related to age or to underlying disease, but was linked to length of hospitalization: 45 per cent of infants hospitalized for one week or more became infected, and the percentage increased with length of stay. Ten of 24 staff members also acquired the virus and appeared to play a major role as virus carriers. Nosocomial respiratory syncytial virus infection poses a major risk for hospitalized infants and adds to hospital costs.