Data on illness by cause are reported in a sample population study in Baltimore during the 5-yr. period, 1938-1943. In each sex, chronic illness annually constituted only about 16% of all illness in terms of cases, but received slightly more than one-third of all medical calls, and was the cause of 24-36% of all hospital admissions. There was a real difference in the incidence of acute illness among males and females when female genital causes of illness were excluded. The annual rates were (male) 1037 and (female) 1387 per 1000. The difference was evident chiefly in the minor respiratory diseases which constitute slightly more than 50% of the illness in each sex group. No such marked differences between sexes were observed for medical calls or for hospital admissions for acute illnesses. The prevalence of major chronic disease was considerably higher among females than among males. The annual prevalence per 1000 was 160 for males and 244 for females. 14% of the population observed in the 3d year of the morbidity study had one or more major chronic illnesses. The mean annual incidence of new cases of the major chronic diseases was 24 per 1000.