In a clinical context, the term "overweight" usually connotes adiposity, an excess of body fat. In the absence of any direct or simple indirect methods of measuring total body fat, medical research workers often use formulae (indices) which relate body weight to stature. So far as we know, the uses and limitations of such formulae have never been critically examined. It is obvious that no formula relating weight to height can measure adiposity; the most we can hope for is that values of the index will be highly correlated with adiposity. The index cannot distinguish between heaviness due to adiposity, muscularity, or oedema, and, if adiposity is in question, should not be used in investigations where differential water retention may have an important influence on body weight, nor to compare groups of unusually muscular persons, such as athletes, with groups not so selected. Given these limitations, an index should provide a convenient and objective way of selecting for more detailed study, groups in which weight is unusually high or low. It should also be useful as a basis of standardization, and as a variable in mul tiple regression analyses. To perform these functions satisfactorily an index should have the following characteristics: