Chest physiotherapy is recognized as a mainstay of cystic fibrosis therapy, and is particularly important during treatment of pulmonary exacerbations. For some patients, especially those with moderate to severe compromise of lung function, this therapy may impose a high demand in the work of breathing and energy expenditure. The goal of this study is to evaluate the effectiveness of adding pressure support ventilation (PSV) during chest physiotherapy sessions in patients with cystic fibrosis with moderate to severe lung function compromise being treated for a pulmonary exacerbation.