Background Regular physical activity (PA) and an active lifestyle are associated with health benefits and increased life expectancy in patients with cystic fibrosis (CF). The introduction of new modulator therapy may influence PA levels by improving patient’s functional health status. This study aimed to investigate whether Elexacaftor/Tezacaftor/Ivacaftor (ETI), had an impact on daily PA levels in patients with CF. Methods 18 patients (7 males, 11 females) participated in the study. PA level and lung function data were measured objectively, with accelerometry (for 7 consecutive days) and spirometry respectively at: baseline (T0), 1 month after starting ETI (T1) and between 3 and 6 months post-ETI administration (T3+). Results The mean relative forced expiratory volume in 1 second (FEV1%pred) significantly increased from T0 (68.4±18.1) to T1 (78.2±21.2, p<0.05). The mean relative forced vital capacity (FVC%pred) significantly increased from T0 (88.2±14.9) to T1 (94.0±13.9, p<0.05). PA parameters at T0 were: 6460.2± 2068.8 steps/day and 46.8±35.0 min/day for moderate to vigorous PA (MVPA) time. None of the parameters presented statistical differences throughout all the measurements compared to T0. The mean steps/day were 6253.8±2253.5 at T1 and 7036.0±3177.0 at T3+. The mean MVPA time (min) were 36.2±15.6 at T1 and 48.9±24.4 at T3+. Conclusions Despite the observed improvement in lung function, daily PA levels in patients with CF do not show a significant enhancement after ETI treatment. This underscores the importance of ongoing support and encouragement for physical activity in patients with CF, even when undergoing modulator treatment. Implementing PA promotional programs remains crucial to fostering an active lifestyle in individuals with CF.