A 24 hour energy balance study was carried out on 20 black Kenyan children with acute measles and repeated after recovery. The energy content of a weighed 24 hour food intake and of a simultaneous collection of faeces and urine was determined by bomb calorimetry. Energy expenditure was measured by indirect calorimetry using a purpose built flow over calorimeter. The nutritional state of the children was assessed by anthropometry at the time of each study and during convalescence. The results showed a fall of roughly 75% in the intake of gross and metabolisable energy during measles, while the resting energy expenditure was little affected. Thus the severe degree of negative apparent energy balance observed during measles is the combined effect of underfeeding in ill children, and failure, during starvation related to infection, of the early fall in metabolic rate that characterises simple underfeeding.