AbstractAnaerobic fitness is important for daily functioning of children with cystic fibrosis (CF). The aim of this study was to assess the determinants of anaerobic performance in CF. Anaerobic performance was measured in 39 children with CF (mean age, 13.2 ± 1.8 (SD) years, forced expired volume in 1 sec (FEV1) 81.6 ± 22.1% predicted), using a Wingate anaerobic test. Significant associations were found for peak power (PP) and mean power (MP) with fat‐free mass (FFM) body weight, body mass index, maximal isometric muscle force, and aerobic capacity. Pulmonary function was correlated with anaerobic indices when controlled for FFM. Multiple regression analysis indicated that FFM and FEV1 accounted for 82% and 86% of the variability in PP and MP, respectively. Patients with moderate CF (FEV1 < 80%), as compared to mild CF (FEV1 ≥ 80%), had higher PP (difference = 85 W, 95% CI = 27–144 W) and MP (difference = 53 W, 95% CI = 42–63 W) at equivalent FFM. Our results indicate that FFM and pulmonary function are important determinants of anaerobic exercise performance in children with CF. With progression of pulmonary disease, anaerobic performance may be enhanced. Pediatr Pulmonol. 2003; 36:223–229. © 2003 Wiley‐Liss, Inc.