The Flutter VRP1 (Flutter) is claimed to increase the clearance of excess bronchial secretions and to improve lung function and oxygenation, but these claims were based on computer models, laboratory assisted experiments and uncontrolled clinical trials. A prospective randomized clinical trial was undertaken, in subjects with cystic fibrosis, using the active cycle of breathing techniques as the 'gold standard'. The claims could not be substantiated and the possibility of sputum retention was of concern when the Flutter was used as significantly more sputum (P < 0.001) was cleared with the active cycle of breathing techniques alone. There were no significant changes in lung function or oxygenation.