The median age of survival in patients with cystic fibrosis (CF) has improved considerably. Despite this improvement, deterioration of pulmonary function and decrease in exercise capacity are still the main problems for many patients. Although dyspnoea is a common complaint in CF patients, relatively little regard has been paid to this symptom. This study examined the relationship between dyspnoea, bicycle exercise capacity and pulmonary function in patients with CF. In 14 patients in a stable clinical condition, pulmonary function [forced expiratory volume in 1 s (FEV1), inspiratory vital capacity (IVC)], bicycle exercise capacity [maximum exercise capacity (Wmax)], subjective degree of dyspnoea during daily living [Medical Research Council (MRC) dyspnoea scale], and during exercise (Borg scale) were assessed. The mean (SD) age of the patients was 25 (6.8) years, FEV1 was 41 (19)% predicted, IVC was 63 (17)% predicted and FEV1/IVC ratio was 47 (10)%; median (range) Wmax was 55 (0-79)% predicted. Bicycle exercise test performance appeared to be mainly determined by pulmonary function and MRC dyspnoea grade; multiple regression equation containing FEV1 and dyspnoea accounted for 76% of the variance in Wmax (% predicted) (Wmax = -7.9 dysp + 1.1 FEV1, +24). Exercise dyspnoea, assessed by the Borg scale, showed a significant linear correlation with minute ventilation. (VE), maximal voluntary ventilation (MVV) (%) (r = 0.76; P < 0.001). Medical Research Council dyspnoea score correlated relatively poorly with FEV1 (% predicted) (r = -0.17; n.s.) and IVC (% predicted) (r = -0.48; n.s.). Borg score at maximal exercise did not correlate with MRC dyspnoea score (r = -0.07). Borg50% score correlated significantly with MRC dyspnoea score (r = 0.61; P < 0.05). These results show that dyspnoea has an influence on exercise capacity. Dyspnoea score showed a large inter-individual variation, not strongly related to pulmonary function. It is concluded that dyspnoea deserves more attention in CF patients and needs to be assessed in rehabilitation programmes and other intervention studies in these patients.