O'Donoghue, J. M., C. G. Ray, D. W. Terry, Jr. and H. N. Beaty (Harborview Medical Center, 325 9th Ave., Seattle, Wash. 98104). Prevention of nosocomial influenza infection with amantadine. Am J Epidemiol 97: 276–282, 1973.—The prophylactic effectiveness of amantadine hydrochloride in preventing hospital acquired influenza infection was evaluated during a community outbreak of influenza A (H3N2)infection. All patients admitted to a hospital medical and neurology service over a 30-day period were included in the study. One hundred-eleven patients fulfilled study criteria. Fifty received amantadine and 61 were untreated. A total of 14 patients acquired either clinical or subclinical infection. Seven clinical and five subclinical infections occurred in patients who did not receive amantadine, while only two subclinical infections occurred in patients receiving treatment. In terms of preventing infection, amantadine prophylaxis resulted in a 79.7% protective effect, and serious consideration should be given to its use in hospitalized patients during future influenza A outbreaks.