New criteria are proposed for classifying chronic energy deficiency (CED) in adults. A progressively more precise approach to identifying affected individuals involves measuring body weight and height, then energy intake (or expenditure) and finally the basal metabolic rate (BMR). Three cut-off points for body mass index (BMI) were identified: 18.5, 17.0 and 16.0. A BMI above 18.5 is classified as normal and below 16.0 as grade III CED. A diagnosis of grades I and II CED depends on finding the combination of a BMI of 16.0-16.9 or 17.0-18.4 with a ratio of energy turnover to predicted BMR of less than 1.4. Measuring the individual BMR avoids misclassification and confirms the diagnosis. In groups of African adults 38-63 per cent of each group had a BMI below 18.5 and the majority require studies of their energy turnover before specifying their degree of CED; 3 per cent of Ethiopian women and 24 per cent of a selected male African group had grade III CED. These guidelines can be used when assessing the input of aid programmes and for clinical and other studies.