Maximum oxygen consumption (VO2 peak) during cardiopulmonary exercise test (CPET) is commonly used to assess working capacity in patients with cystic fibrosis (CF). The determinants of oxygen consumption after exercise in these patients are unknown. We assessed indices of oxygen kinetics during CPET and early recovery and explored their relation to disease severity measured by Schwachman’s score (SS). Nineteen patients with CF (10 male/9female, aged 24±13, SS 71±14) and 11 healthy subjects (3 male / 8 female aged 29±4) underwent a maximum CPET on a treadmill. Breath by breath analysis was used for measuring oxygen consumption (VO2), carbon dioxide production (VCO2) and ventilation. Maximum oxygen consumption (VO2 peak) and early recovery first degree slope (VO2/t-slope) for VO2 were calculated. Peak oxygen consumption was significantly lower in CF patients compared to healthy subjects (21±8 vs 35±8 ml/kg/mI p<0.01). In the CF group, VO2/t-slope correlated to VO2 peak (r=0.92, p<0.001), as well as to SS (r=0.86, p<0.001). We conclude that in CF patients, the prolonged oxygen kinetics during early recovery from maximal exercise is related to the disease severity. Pneumon 2000, 13 (2): 137-143