Cystic fibrosis is an autosomal recessive hereditary disease which has a progressive course and affects numerous epithelial organs. The aim of the study was to evaluate the effectiveness of treatment by triple therapy of elexacaftor/tezacaftor/ivacaftor in adult patients with cystic fibrosis. We explored the connection between indicators of lung function, body mass index and body composition estimated by bioimpedance before starting and after a minimum of six months of therapy with CFTR (cystic fibrosis transmembrane regulator) modulators, while monitoring possible side effects and complications of said therapy. 44 patients with cystic fibrosis who are treated at the Center for Cystic Fibrosis of Children and Adults at the Zagreb Clinical Hospital Center participated in the research. Demographic, anthropometric and clinical data were collected (pulmonary function tests using spirometry and results of bioelectrical impedance analysis using a Tanita scale). The analysis showed a significant increase in all monitored lung function values after taking the therapy and a decrease in chronic lung colonization by the most common pathogens. The results showed an increase in body mass index and the parameters of body composition analysis by bioimpedance. The triple therapy reduced the number of hospitalizations, pulmonary exacerbations and antibiotic treatment and indicated a good safety profile without severe side effects. The elexacaftor/tezacaftor/ivacaftor triple therapy significantly improves the clinical picture of a patient with cystic fibrosis and it undoubtedly improves the patient's quality of life.