BACKGROUND: Endotracheal intubation and prolonged immobilization of patients receiving mechanical ventilation may reduce expectoration function. High frequency chest wall oscillation (HFCWO) may ameliorate airway secretion movement; however, the vigorous oscillation may influence ventilator settings and change instantaneous cardiopulmonary responses. The aim of this study was to investigate these issues. METHODS: Seventy-three patients aged >20 years who were intubated with mechanical ventilation for pneumonic respiratory failure were recruited and randomly classified into two groups (HFCWO group, n=36; and control group who received conventional chest physical therapy (CCPT), n=37). HFCWO was applied with a fixed protocol, while CCPT was conducted using standard protocols. Both groups received sputum suction after the procedure. Changes in ventilator settings and the subjects' responses were measured at pre-set intervals and compared within groups and between groups.