BACKGROUND: Physical activity (PA) and sedentary behavior (SB) have marked impact on key prognostic indicators such as aerobic capacity and lung function in people with cystic fibrosis (CF) and may have associations with sleep, well-being, and health-related quality of life (HRQOL). METHODS: This observational study assessed PA, SB, aerobic capacity, spirometry, sleep, well-being, and HRQOL in adults with CF at University Hospital Limerick. PA and SB were assessed using an accelerometer that was worn for 7 days. A cardiopulmonary exercise test assessed aerobic capacity. Spirometry was performed according to American Thoracic Society guidelines. Well-being was measured by the AWESCORE, sleep quality by the Pittsburgh Sleep Quality Index (PSQI), and HRQOL using the CF QuestionnaireRevised. RESULTS: Thirty-three participants (13 males/20 females) were recruited. Mean age was 26.2 y (+/- 7.1 SD), with mean FEV1 72.9% of predicted (+/- 26.2 SD). Mean step count was 7,788 (+/- 3,583 SD). Over 75% of participants did not reach recommended PA targets ( 10,000 steps), with females being 25.5% less active than males. The PSQI indicated 48.5% of participants scored > 5, indicating poor sleep quality. Number of steps and SB demonstrated a moderate significant correlation with FEV1 (r = 0.45, P = .030; r = -0.37, P = .043, respectively) and sleep quality (r = -0.85, P < .001; r = 0.77, P < .001, respectively). (V) over dotO(2) peak expressed relative to body weight, and as a percentage of predicted, was significantly positively correlated with step count (r = 0.48, P = .007; r = 0.42, P = .02, respectively) but did not correlate with SB (P = .96). (V) over dotO(2) peak (L/min) strongly correlated with FEV1 (r = 0.75, P < .001). CONCLUSIONS: Most participants did not meet PA targets. PA levels correlated to aerobic capacity, FEV1, and self-reported sleep quality, and this should be considered in longitudinal studies and in PA interventions. Key words: Cystic fibrosis; physical activity; sleep; well-being. (c) 2022 Daedalus Enterprises