Eleven persons who had previously been patients in a community hospital in Cook County, Illinois, had onset of serum hepatitis in May, June, and July 1969. Three months before developing hepatitis, all 11 had been hospitalized on the same ward and cared for by a nurse who was late in the incubation period of serum hepatitis. The 11 hepatitis patients had no unique exposures to hepatotoxic drugs, blood transfusions, contaminated foods or water, or other hepatitis cases. However, they had been given more inoculations by the infected nurse than did a comparison group of patients selected from the same ward. Although no obvious breaks in aseptic technique were found, it seems likely that the virus was spread from nurse to patients by either parenteral or oral contact. The exact route of transmission, however, was not established.