The methods of meeting a naval emergency—a rapid increase in personnel aboard ship, lengthened periods in the forward areas, sacrifice of personnel space in favor of ordnance—all place considerable stress on human performance. One way in which these stresses may manifest themselves is by increasing the number of incapacitations from disease and accidents. Consequently, a quantitative knowledge of this effect may help to determine to what extent increasing the fire power of a vessel at the expense of its personnel is in the long run a profitable venture. This is a question of some practical importance, and, in addition, the same quantitative information is of theoretical interest insofar as it sheds light upon the mechanism whereby stress takes effect on a human population. For these reasons it was thought worth while to undertake a study of the effect of combat, overcrowding, and weather on the incidence of certain diseases and accidents aboard several Fleet vessels.