Lifelong physical activity is an important component of the therapeutic management of patients with cystic fibrosis (CF). Use of the internet to monitor and encourage participation in physical activity has not been assessed in adults with CF. We aimed to establish the feasibility and acceptability of a specifically developed internet-based program to monitor and encourage physical activity participation in adults with CF.Subjects were recruited at hospital discharge to trial an internet-based physical activity program (ActivOnline) for 8 weeks, which incorporated fortnightly telephone consultation to support physical activity behavior change. Acceptability of the program was assessed by semistructured interview, as well as subject-rated system usability and perceived benefit using Likert scales. Feasibility was assessed by frequency of access of the online site and number of physical activity sessions recorded.Ten subjects were recruited who rated system usability and perceived benefit favorably (median score usability of 89% [interquartile range of 84-95%]; median score of perceived benefit (maximum of 5) of 4 [interquartile range of 3-4.8]). During interviews, subjects described a positive reaction to receiving graphical representation of their activity participation; however, 80% would have preferred a more mobile interface such as an app. Subjects accessed ActivOnline on a mean ± SD of 13 ± 11 occasions over 8 weeks and recorded a mean of 35 (range of 15-57) physical activity sessions.Use of an internet-based program to encourage participation in physical activity was both feasible and acceptable to adults with CF. Feasibility may be further improved with the ability to access the program through a mobile application.