We examined aerobic and anaerobic exercise performance in 17 subjects with cystic fibrosis (CF) (age 25 ± 10 [SD] yr; 47% females; FEV1 62 ± 21% pred) and 17 age- and sex-matched control subjects (age 25 ± 8 [SD] yr; 41% females; FEV1 112 ± 15% pred) in relation to pulmonary function and nutritional status. Aerobic capacity was determined as maximal oxygen consumption (V̇ o2max) (ml/kg/min) and anaerobic threshold (AT; ml V̇o 2/kg/min) from a graded exercise stress test on an electronically braked bicycle ergometer. Anaerobic performance was assessed from the average work of two bouts of pedaling to exhaustion at a load corresponding to 130% V̇o 2max from graded exercise. Both aerobic and anaerobic performances were decreased in subjects with CF (p < 0.001). The duration of anaerobic exercise in subjects with CF was similar to control subjects. In control subjects, pulmonary function did not correlate to aerobic or anaerobic exercise. In subjects with CF significant relationships between FEV1, vital capacity, and FEF25–75% to AT were found, suggesting the pulmonary limitation to aerobic capacity. In both patients with CF and control subjects, lean body mass and arm muscle area significantly correlated with anaerobic performance but not with V̇o 2max or AT. We conclude that nutritional status, rather than pulmonary function, is the major determinant of anaerobic exercise capacity in CF. The preserved duration of anaerobic exercise at equivalent workloads (corresponding to 130% of V̇o 2max from graded exercise) suggests that readily available energy stores in muscle may be similar in CF and normal individuals.