Forty-one adolescent males (11.1-18.3 yr) with cystic fibrosis (CF) and 37 healthy adolescent males (11.1-17.9 yr) performed a Wingate Anaerobic Test(WAnT). The group with CF was subdivided by sexual maturity, nutritional status, and degree of airway obstruction. The subjects with CF had lower absolute power outputs than the healthy controls [mean power in Watts (mean± SD): 350.2 ± 135.9 vs 424.5 ± 120.4,P< 0.001; peak power: 525.2 ± 178.4 vs 665.9 ± 191.3,P< 0.001). When absolute power was corrected for lean body mass, the subjects with CF had lower power outputs than the healthy controls (mean power in W·kg-1: 8.9 ± 1.7 vs 9.6 ± 0.9,P< 0.05; peak power: 13.4 ± 2.1 vs 15.0 ± 1.6,P< 0.05). The subgroup with CF with a higher body mass index (BMI > 17.5 kg·m-2) had higher peak and mean power output than subjects with CF with a lower BMI in both absolute power and when power was expressed per lean body mass. When sexual maturation was considered, subjects with CF with salivary testosterone greater than 4.0 ng·dl-1had a higher mean and peak power in both absolute terms and relative to lean body mass than subjects with CF with salivary testosterone less than 4.0 ng·dl-1. Multiple regression analysis indicated that the nutritional factor accounted for 70%-80% of the variability in power output in the subjects with CF, while testosterone accounted for 10% of the variability. Pulmonary function was not a significant independent correlate of anaerobic power. Our results suggest that nutritional status, and to a lesser extent maturational factors, may play a more important role than pulmonary function in determining anaerobic fitness in male adolescents with CF.