1. We investigated the relationship between circulating tumour necrosis factor-alpha concentrations, resting energy expenditure, cachexia and altered intermediary metabolism in patients with cystic fibrosis and chronic pulmonary infection. 2. Twenty adult patients with cystic fibrosis and chronic bronchial sepsis covering a spectrum of severity of lung disease (forced expiratory volume in 1 s 30-100% of predicted) were compared with 10 age matched, healthy, non-cystic fibrosis subjects. 3. Circulating tumour necrosis factor-alpha, C-reactive protein and neutrophil elastase-alpha 1-antiproteinase complex concentrations were determined simultaneously with glycerol, non-esterified fatty acids, catecholamines, anthropometric indices and resting energy expenditure (ventilated hood method). 4. Weight, body mass index and arm muscle mass were reduced in patients with cystic fibrosis compared with healthy control subjects (P < 0.01), whereas mean resting energy expenditure was increased [121 versus 101% predicted, mean difference 19.2% (95% confidence interval 11.0-27.4%), P < 0.001]. Circulating concentrations of glycerol (P < 0.01), non-esterified fatty acids (P < 0.01), adrenaline (P < 0.05), tumour necrosis factor-alpha, C-reactive protein and neutrophil elastase-alpha 1-antiproteinase complex (P < 0.01) were increased in patients compared with control subjects [tumour necrosis factor-alpha 96.9 versus 24.7 pg/ml, mean difference 72.2 pg/ml [95% confidence interval 27.7-116.7 pg/ml), P < 0.001]. Resting energy expenditure was significantly related to tumour necrosis factor-alpha levels and forced expiratory volume in 1 s. 5. In patients with cystic fibrosis and chronic pulmonary sepsis changes in resting energy expenditure, body composition and intermediary metabolism are consistent with the systemic effects of the host inflammatory response, which may be responsible for cachexia in adult patients. In particular these changes are consistent with the action of tumour necrosis factor-alpha, which was detected in the circulation during a period of apparent clinical stability.