Background/Aim: The patients with cystic fibrosis (CF) are living longer compared to the past, but respiratory failure is still the mostcommon cause of mortality. The aim of this study is to investigate factors associated with severe lung disease in a cohort of adult patientswith CF.Materials and methods: Demographic data, clinical and laboratory findings of the patients aged 18 years and more were collected andthe patients were grouped according to forced expiratory volume in 1 s (FEV1) as severe group: <40% and nonsevere ≥40%. Associationswere investigated between groups and clinical outcomes.Results: A total of 76 patients were enrolled in the study. The mean age was 24.5 ± 5.25 years and 36 (47.4%) patients were female. In thesevere group; the mean age was higher (27.1 ± 6.0 vs 23.6 ± 4.7, P = 0.013), the median Chrispin-Norman score of severe lung diseasegroup was higher (14 (6–22) vs 5.5 (0–20), P < 0.001), hospitalization at least once in a year for intravenous antibiotic was more common(12/18 (66%) vs 19/58 (32%), P = 0.014). There was a positive correlation between body mass index (BMI) and lung function, indicatingthat lower nutritional status was related to lower FEV1, r2 = 0.21, P < 0.001. The median FEV1% was lower in patients with CF-relateddiabetes (38 (14–95) vs 66 (13–121), P = 0.042). Dornase alpha use and physiotherapy rate were higher in severe lung disease group (P= 0.008 and P < 0.001, respectively).Conclusion: Lower BMI, older age, presence of CF-related diabetes, higher radiologic scores, use of dornase alpha and physiotherapyand higher hospitalization rate for intravenous antibiotic therapy are significantly associated with severe lung disease.