An outbreak of respiratory syncytial virus (RSV) infection occurred among 31 patients in a marrow transplant center over a 13-week period beginning in January 1990. RSV infection was also documented in 35 family members and employees. Of 18 patients with pneumonia, 14 (78%) died. None of 13 with upper respiratory infection died. Preengraftment patients tended to develop pneumonia more frequently than did engrafted patients. Early administration of ribavirin may have had a beneficial effect in patients with pneumonia. Antigenic and genomic analysis of 14 available isolates suggested that at least four different viral strains were responsible for the outbreak. One group of patients and 1 employee in spatial proximity were infected with the same strain and likely acquired their infections nosocomially. RSV infection in marrow transplant patients is a serious and life-threatening infection with a high mortality rate once pneumonia develops.