Resting energy expenditure (REE) increases during pulmonary exacerbation by Pseudomonas aeruginosa in cystic fibrosis (CF) patients, and decreases after i.v. anti-Pseudomonas aeruginosa antibiotic therapy (IVAT). However, the impact of IVAT on total energy expenditure (TEE) is unknown. The aim of this study was to assess the changes in TEE and its main components after IVAT administered at home. Body composition measured by skinfold thickness and bio-impedance analysis, energy intake (EI) assessed by a weekly diary, REE measured by indirect calorimetry (IC), TEE assessed by a technique using 24-h heart-rate monitoring method and physical activity (PA) monitored using an activity diary (AD) were assessed in 16 patients (9 boys and 7 girls) aged 12.1 +/- 2.3 y (range, 7.1-14.6 y), before and after 28 +/- 4 d including a 14-d IVAT course. After IVAT, weight increased significantly by 1.9% (32.1 +/- 7.5 versus 32.7 +/- 7.6 kg; p < 0.05), while fat mass and fat free mass increased non significantly. EI increased by 4.6% (10,797 +/- 3039 versus 11320 +/- 3074 kJ/d; p < 0.05). TEE was not affected by IVAT (7014 +/- 1929 versus 7081 +/- 1478 kJ/d) whereas REE decreased by 4.1% (5295 +/- 909 versus 5093 +/- 837 kJ/d; p < 0.05), resulting in 9.3% increase in PA assessed by AD converted to metabolic equivalent tasks (MET) (37.0 +/- 3.1 versus 40.7 +/- 4.5 MET; p < 0.05). The improvement in nutritional status after IVAT is not related to a decrease in TEE, but probably to an increase in EI and a decrease of REE after IVAT. After IVAT, the reduction in REE is probably compensated by an increase in PA in CF patients.