Studies on hospitalized elderly subjects have demonstrated that negative energy balance is common during hospitalization, but have concentrated primarily on long-stay and psychogeriatric patients. There is little information on energy balance in elderly patients admitted with acute illness from the community, despite the importance of this patient group and the presence of a number of factors likely to predispose such patients to negative energy balance. In the present study energy balance was quantified in twenty patients (eight males, mean age 82 (SD 5) years; twelve females, mean age 84 (SD 6) years) admitted from the community with acute illness, and predicted basal metabolic rate (BMR) was compared with measured resting metabolic rate (RMR). Most patients were in negative energy balance during hospitalization, and median measured energy intake (EI): measured RMR ratio was 1.0 (range 0.7-1.8). The mean difference between measured EI and estimated total energy expenditure was -1.3 MJ/d (range -3.4 to +2.5 MJ/d). Estimated total energy expenditure exceeded measured EI in fifteen of the patients and there was a significant decline in mid-arm muscle circumference (paired t, P < 0.05) during hospitalization. We conclude that moderate negative energy balance is common in this patient group, and that these patients are at risk of undernutrition during their hospital stay.