In and out of government, determining the cost of illness is a major concern. The allocation of health care resources and the evaluation of current research and program efforts depend in large measure on such information. This article updates the 1963 benchmark study of the cost of illness. For the 16 major diagnostic categories of illnesses, the cost is presented in terms of the direct costs for prevention,detection, and treatment; the morbidity losses due to disability; and the mortality losses resulting from premature death. The method of calculating the cost of any illness is described, and data necessary for the calculation are provided. In 1972, the estimated total cost of illness was $188 billion: $75 billion for direct costs, $42 billion for morbidity, and $71 billion for mortality. Diseases of the circulatory system were the most costly, representing about one-fifth of all costs of illness.