OBJECTIVES: Cystic fibrosis (CF) is an inherited, incurable disease whose mortality today is determined by the progression of lung disease. Lung damage causes the increase in energy needs for the process of breathing which creates difficulties in maintaining adequate nutritional status. The aim of this study was to assess the body composition of adult patients with CF in Croatia. For this purpose, the bioelectrical impedance analysis (BIA) was used and it is also increasingly used today in assessing the body composition of chronic patients due to its simplicity and accuracy. ----- MATERIALS AND METHODS: We retrospectively collected data from the Hospital Information System of the University Hospital Center Zagreb on 30 adult patients suffering from CF who were treated in the Center for Cystic Fibrosis of Children and Adults at the University Hospital Center Zagreb in 2018. Basic demographic, anthropometric and clinical data such as pulmonary function tests and BIA results were collected. ----- RESULTS: The study involved 30 adult patients, of whom 17 were women and 13 men ranged in age from 17 to 34. The median body mass index (BMI) was 21 kg/m2. The medians of the BIA parameter results were: FFM=45,80 kg, FFMI=5,9 kg/m2, SI=9,45 m2, PhA=5,9°. We found statistically significant correlations between: FVC and the number of outpatient exacerbations (F=6,477; p=0,0167), FEV1 and FFMI (r=0,49; p=0,005; 95% CI 0,16 – 0,72), FEV1 and SI (r=0,54; p=0,001; 95% CI 0,23 – 0,75). We also demonstrated a statistically significant effect of sarcopenia on pulmonary function (p=0,04). ----- CONCLUSION: A BIA gives us more information about body composition than BMI. FFM and FFMI give us especially important information because they indicate the amount of skeletal muscles that play an important role in maintaining lung function. In our study, we demonstrated a statistically significant effect of sarcopenia on lung function in adult CF patients in Croatia, whereby our female patients had better FFM and FFMI values compared to male patients which could be the reason for their numerical predominance in the total number of adult patients with CF in Croatia.