The majority of patients with cystic fibrosis (CF) will grow into adulthood. Despite this improvement in survival, the disease is progressive, especially with respect to the decline in pulmonary function, which in turn may have an impact on a patient's quality of life. In this study we evaluated the quality of life in CF adults and examined the relationship between quality of life and pulmonary function, exercise capacity, and dyspnea. We assessed in 15 patients in stable clinical condition their forced expired volume in the first second (FEV1) inspiratory vital capacity (IVC), cycle exercise capacity (Wmax), and subjective degrees of dyspnea during daily living (MRC dyspnea scale). Quality of life was assessed with the Sickness Impact Profile (SIP). A sample of 100 healthy individuals, ranging in age from 18 to 30 years, served as the control group. Mean (SD) age of the patients was 25.9 (7.3) years, FEV1 was 38 (16)% predicted, IVC was 65 (17)% predicted, FEV1 IVC ratio was 46 (10)%, and Wmax was 90 (54) W. The overall SIP and physical SIP scores in CF patients were significantly higher than in the controls, indicating more impairment in overall and physical functioning in the patients than in the control group (P < 0.001). The psychosocial SIP score did not differ significantly between the two groups. Overall SIP score correlated poorly with FEV1 % predicted (r = -0.33; n.s) and IVC % predicted (r = -0.36; n.s.) but showed a better and significant relationship to the maximal exercise capacity (r = -0.57; P < 0.05). MRC dyspnea scores showed a strong correlation with overall SIP scores (r = 0.75; P < 0.001). These results show that CF affects quality of life in adults primarily due to a limitation in physical functioning. Psychosocial functioning did not differ from that of healthy controls. Exercise capacity and dyspnea scores were related to the impairment in the quality of life. Therefore, the effects of programs aimed to improve exercise capacity and reduce dyspnea on CF patients' quality of life need to be evaluated.