BACKGROUND AND AIMS: Undernutrition is common in young adult patients with cystic fibrosis (CF) and implies an imbalance between energy intake and total energy expenditure (TEE). The aim of this study was to measure energy intake and TEE expenditure in a group of patients when they were clinically stable at home and during an exacerbation of respiratory symptoms when they were in hospital receiving intravenous antibiotics. METHODS: Eleven patients aged between 15 and 40 years with moderate respiratory disease, mean FEV(1) 51.4% predicted took part. An exacerbation was defined as>15% decrease in FEV(1), an increase in sputum production and a decision to treat in hospital with intravenous antibiotics. Resting energy expenditure (REE) was measured using indirect calorimetry and energy intake by 3 day food diaries. TEE expenditure was measured using 24 h heart rate and doubly isotopically labelled water. RESULTS: REE was higher at the beginning of an exacerbation than the end, P<0.05. There was no significant difference in TEE during the stable period compared to the exacerbation 10.53(2.39)MJ/day compared to 8.77(1.59)MJ/day using doubly isotopically labelled water. There was no difference in energy intake during the exacerbation compared to the stable period, 11.19(2.31)MJ/day compared to 11.77(2.30)MJ/day. CONCLUSIONS: There was no difference in TEE and energy intake when patients were unwell in hospital compared to when they were stable at home despite an increase in REE.