This study examined the extent that pulmonary function is related to perceived health status and global quality of life in adults suffering from cystic fibrosis, and the extent that self-efficacy modifies these relationships. Our sample comprised 86 adults (48% female; mean age, 29 years; age range, 18-54 years) with cystic fibrosis, recruited from the Norwegian Competence Centre for Cystic Fibrosis (83% response rate). Both subjective data on psychosocial aspects of living with cystic fibrosis, and objective data on pulmonary function (FEV1%) were assessed. Psychosocial aspects were assessed by a questionnaire that included St. George's Respiratory Questionnaire (perceived health status), the Generalized Self-Efficacy Scale (perceived self-efficacy), and the Quality of Life Scale (global quality of life). We found that self-efficacy is an additional factor explaining health status and global quality of life. Pulmonary function is important for health status, but not for global quality of life, for which perceived self-efficacy and health status are the most significant variables. These results show that self-efficacy is as important as pulmonary function for the perceived health status and global quality of life of patients. These valuable findings should be considered in the treatment and care of adult patients suffering from cystic fibrosis.