Objectives: Assuming a causal relationship between depression and the progression of CF, the aim of our study was to show that elevated symptoms of depression could be a predictor of worse lung function.Methods: 22 CF patients were recruited form our adult CF centre (13 women, 9 men, mean age 24, 18-33 years).The patients completed Patient Health Questionnaire-9 (PHQ 9) and the Generalized Anxiety Disorder 7item scale (GAD 7).Body mass indeX (BMI) and pulmonary function testing were measured.Results: Level of anxiety and depression showed no significant association with decreased lung function.This can be explained by the fact that none of the subjects had serious anxiety and/or depression and prospective study should bed one. Conclusion:The interaction between depression and disease progression could be linked to the lack of hope, optimism and perceived self-efficacy in depressed patients, consequently inhibiting their fighting spirit against the disease and necessary adherence to treatment.Thus, depressed patients may fail to respond to standard medical treatment due to the nature of their mental health problems, and may experience a greater loss of lung function than necessary.This effect may then reinforce the negative thoughts related to their own prognosis, triggering a vicious psychosomatic cycle.In contrast, patients who are not depressed and already have severely restricted lung function might be more able to make use of resources for their daily treatment routines.This, in turn, might result in stable or even slightly improved outcomes.