Exercise is an important part of normal childhood, but the ability to exercise may be impaired in chronic lung diseases such as cystic fibrosis (CF). Improving exercise performance by training is very attractive. The aim of the present study was the evaluation of the effects of a physical aerobic training program, performed in the Children's Hospital and Research Institute "Bambino Gesù" (Rome, Italy) in outpatient CF children, supervised by a physician. Twelve patients (mean forced expiratory flow in 1 sec (FEV1), 71%), age range 12-24 years (16.7 +/- 4.4 years), were enrolled. They performed a maximal exercise stress test on the treadmill (modified Bruce protocol) with breath-by-breath determination of oxygen consumption (VO2) to maximum at end-exercise; we measured time of exercise (TE), maximal heart rate (Hrmax) in beats per minute (bpm), and maximal systolic blood pressure (SBPm) in mmHg. The program consisted of 12 weeks of training twice a week. Each training session consisted of walking or running on the treadmill for 30 min at the speed that allowed the child to attain 60% of the maximal heart rate obtained during a baseline stress test for 4 weeks, 70% in the following 4 weeks, and 80% in the last 4 weeks, under strict medical supervision. HR was continously monitored. There was no change in FEV1 and forced vital capacity after the treatment period. Hrmax and SBPm also remained the same (P = 0.37 and P = 0.25, respectively). There was a significant increase in TE (P < 0.002), VO2, VO2/kg, and pulmonary ventilation (VE) (P < 0.0001, P < 0.001, and P < 0.001, respectively). This pilot study showed that a simple training program improves short-term cardiopulmonary fitness in children with CF. Further studies with a larger sample and for a more prolonged time are necessary to assess if sport can have a long-term effect on lung function or survival in CF patients.