Introduction. In cystic fibrosis (CF), a thick secret with pathogenic microflora in the respiratory tract and other factors lead to a decrease in lung function, repeated hospitalizations and a decrease in the quality of life of patients.Aim. To evaluate the influence of endogenous and exogenous factors on lung function in children and adolescents with CF with the F508del variant in the homozygous state.Materials and methods. Spirometry parameters were analyzed in 379 patients with the F508del variant in the homozygous state at the age of 6–18 years according to the National Register of CF Patients 2019.Results. In children with CF at the age of 6–10 years, FEV1 and FVC were significantly higher than in children aged 11–14 and 15–18 years. P. aeruginosa and Achromobacter spp. infection had a negative effect on lung function. A weak correlation was found between nutritional status and respiratory function (for FEV1 r = 0.229; p <0.001; for FVC r = 0.206; p <0.001). In the Southern, Urals, and North Caucasian federal districts of the Russian Federation, the parameters of respiratory function in patients were significantly lower than in the Central Federal District.Conclusions. For a group of patients, carriers of the F508del genotype in the homozygous state, factors that negatively affect respiratory function were identified: adolescence, low nutritional status, respiratory tract infections P. aeruginosa, Achromobacter spp., Burkholderia cepacia complex, liver cirrhosis with portal hypertension and features of organizational assistance to patients. Some factors are correctable in the organization of preventive measures, which can have a positive impact on reducing the rate of damage to the bronchopulmonary system. Regular monitoring and timely correction of nutritional status, monitoring of respiratory tract infections, their prevention and timely completion of eradication therapy courses are required. Adolescent patients require special attention.