The six-minute walking test (6MWT) has been widely utilized to evaluate global exercise capacity in patients with cystic fibrosis. The aim of this study was to assess the exercise capacity by 6MWT, measuring four outcome measures: walk distance, oxygensaturation and pulse rate during the walk, and breathlessness perception after the walk, in a group of cystic fibrosis adults with mild to moderate lung disease, and in healthy volunteers, as the control group. Moreover, the study examined the relationship between 6MWT outcome measures and pulmonary function in patients. Twenty-five adults (15 females, age range 18-39 years) with cystic fibrosis and 22 healthy volunteers (14 females, age range 20-45 years) performed a 6MWT following a standard protocol. Walk distance, oxygen saturation (SpO2) and pulse rate at rest and during walk, and breathlessness perception after walk assessed by visual analogue scale (VAS) were measured. Cystic fibrosis patients did notdiffer from healthy volunteers in walk distance (626 +/- 49 m vs. 652 +/- 46 m) and pulse rate. Patients significantly differed from healthy volunteers in SPO2 during the walk (mean SpO2) (P < 0.0001) and VAS (P < 0.0001). In patients, SPO2 during the walk significantly correlated with forced expiratory volume in 1 sec (FEV1) (P < 0.0001), residual volume (RV) (P < 0.001), resting SPO2 (base SpO2) (P < 0.001), and inspiratory capacity (IC) (P < 0.01). In addition, VAS significantly correlated with resting SPO2 (P < 0.01) and IC (P < 0.01). On the basis of regression equations by stepwise multiple regression analysis, SpO2 during walk was predicted by FEV1 (r2 = 0.60) and VAS by IC (r2 = 0.31), whereas walk distance was not reliably predicted by any assessed variables. This study showed that cystic fibrosis adults with mild to moderate lung disease covered a normal walk distance with unimpaired cardiac adaptation, but experienced a significant fall in oxygen saturation and an increased breathlessness perception during exercise. Resting pulmonary function was related to oxygen saturation and breathlessness perception during walk, but contributed significantly only tothe prediction of oxygen saturation. We suggest that 6MWT could be valuable for identifying patients who might experience oxygen desaturation and dyspnoea during demanding daily activities.