Although physical activity may reduce lung function decline in youth with cystic fibrosis (CF), most patients are inactive. Little is known about why youth with CF are inactive or how to facilitate physical activity. This study explored perceptions toward physical activity in 14 youth with CF at a Canadian Hospital. Qualitative interviews were conducted and a grounded theory analysis was undertaken. The participants demonstrated positive or negative perceptions toward physical activity and different experiences--such as parental support and illness narratives--influenced youths' perceptions. In addition, the participants experienced physical activity within the context of reduced time. Recommendations for developing physical activity interventions, including the particular need to ensure that such interventions are not perceived as wasteful of time, are provided. (Contains 1 table and 1 figure.)