† A program of vaccination against poliomyelitis was undertaken among 7,760 Navy families in Hawaii because there was reason to fear that an outbreak, until then concentrated in a single Navy housing area, might spread. The vaccinations were limited to married Navy personnel and their dependents and began Oct. 5, 1955. Epidemiological observations made during the epidemic showed that military and civilian nurseries were important in spreading poliomyelitis among preschool children, from whom the infection spread to their parents, older children, and other adults. After the mass vaccination program, attack rates were lower in vaccinees than in the unvaccinated population, although this difference was not statistically significant. Epidemiological and clinical data did not reveal any evidence of a provoking effect due to injections of poliomyelitis vaccine.