An important aspect of obesity is its multiple causes. We are now on the threshold of understanding a few of these for the first time. Recent investigations have done much to unearth the evidence on the various factors.1 The discovery of a fat-mobilizing or adipokinin activity in a relatively pure corticotropin preparation2 opens other vistas. These studies leave the feeling that one is dealing with a new century of thought on this subject. The concept that obesity is merely the result of overeating, or of an inflow of energy greater than outflow,3 presupposes (1) that the obese has normal physiological responses and (2) that the person is a simple storage tank. Obesity now appears to be a qualitative physiologic change, metabolic and hereditary, in which the regulatory mechanisms of fat deposition * and conversion may be at fault. A distinction, perhaps, should be made between obesity, a severe,