Background. Exercise tolerance is compromised in many patients with cystic fibrosis (CF) and negatively affects their quality of life. The six-minute walk test (6MWT) can be used to assess exercise tolerance. Aim. To investigate whether cystic fibrosis transmembrane conductance regulator (CFTR) modulators are associated with an improvement in exercise tolerance. Methods. This is a retrospective study including children aged ≥ 5 years who began treatment with lumacaftor/ivacaftor (LUM/IVA) or elexacaftor/tezacaftor/ivacaftor (ELX/TEZ/IVA) at the Cystic Fibrosis Centre for Children and Adults at the University Hospital Centre Zagreb in the period between 15. 10. 2021. and 30. 10. 2022. Patients were monitored during the first 12 months of treatment; every 3 months body mass index (BMI) was calculated and spirometry and 6MWT were performed. Collected data were analysed for a change in 6MWT results, BMI and percent-predicted FEV1 (ppFEV1) as well as correlation between these parameters. Missing values were replaced using imputation methods. Results. 36 patients (median age 14 years) were included; 27 received ELX/TEZ/IVA and 9 received LUM/IVA. Average 6MWT walking distance (6MWD) prior to treatment and after 12 months were 522.1 ± 99.9 and 592.0 ± 106.3 m respectively (p < 0.001). Average increase in 6MWD in the first half-year was significantly higher than in the following 6 months (p = 0.023). Changes in BMI z-score (p = 0.006) and ppFEV1 (p < 0.001) during treatment were also significant. 6MWD at 12 months positively correlated with BMI z-score (p = 0.002) and ppFEV1 (p < 0.001). Conclusions. Both clinically and statistically significant improvement in 6MWT results was visible following initiation of CFTR modulator treatment. A positive correlation with lung function improvement and improvement in BMI was observed.