Starting in December of 1965, thirty-one infants, most of them African American, from Washington, DC, received an investigational respiratory syncytial virus (RSV) vaccine as part of a series of clinical trials. Not only did the vaccine not protect the children, who were aged 2 to 7 months when the study began, but it made them more vulnerable to serious illness when they were naturally exposed to the virus. Eighteen of the 23 vaccinated children who became infected, but only 1 of 21 infected children in a control group, subsequently were hospitalized with RSV infection and 2 toddlers in the RSV vaccine group—a 14-month-old and a 16-month-old—died.