Polyvalent oil-adjuvant influenza vaccine has been shown to be practical and effective in children. Its use resulted in respectable levels of antibody after 2 and 3 years when two injections were given. A protection ratio of 3.1 was observed during natural challenge with A2 virus after 2½ years. The value of a program of surveillance for the presence of viral respiratory disease in a hospital population offers the opportunity to test vaccine efficacy under conditions of natural challenge.