1. Maximal exercise capacity in cystic fibrosis is influenced by both pulmonary and nutritional factors: lung disease by limiting maximal achievable ventilation, and malnutrition through a loss of muscle mass. The associated reduction in everyday activities may result in peripheral muscle deconditioning. 2. We studied 14 stable patients with cystic fibrosis (six males, eight females) and 14 healthy control subjects (seven males, seven females) in order to assess the influence of these factors on exercise performance. Subjects underwent anthropometry to estimate muscle mass, spirometry to assess ventilatory capacity, a 30 s sprint on an isokinetic cycle ergometer to assess maximal leg muscle performance, and progressive cycle ergometry to assess overall exercise capacity. 3. Compared with control subjects, the patients with cystic fibrosis were of similar age and height but weighed proportionately less [% ideal weight (mean +/- SD): 94.3 +/- 9.64 versus 109.5 +/- 11.82] and showed evidence of airflow limitation [forced expiratory volume in 1.0 s (FEV1.0) 72.5 +/- 24.78 versus 112.6 +/- 14.25% of predicted]. 4. The patients with cystic fibrosis did less absolute (5.1 +/- 1.89 versus 7.3 +/- 1.97 kJ) but similar relative maximal (11.5 +/- 3.41 versus 13.1 +/- 3.55 kJ/kg lean body mass) sprint work. During progressive exercise, the group with cystic fibrosis achieved lower absolute [maximal O2 consumption (VO2max.) 1.8 +/- 0.527 versus 3.0 +/- 0.655 litres/min] and relative (VO2max./kg lean body mass: 40.5 +/- 9.23 versus 53.0 +/- 11.62 ml min-1 kg-1) work levels.(ABSTRACT TRUNCATED AT 250 WORDS)