Objective To evaluate the effect of continuous aspiration of subglottic secretions (CASS) on the prevention of ventilator-associated pneumonia (VAP) in mechanically ventilated patients. Methods Patients ventilated mechanically at the ICU from October, 2004 to April,2006 were randomly divided into 2 groups: one group received CASS and the other did not (NASS group). CASS was performed immediately after admission for patients in the CASS group. The diagnosis d VAP was made based on clinical presentations, and the evaluation of YAP was done using simplified version of the clinical pulmonary infection score (CPIS). The general status of the patients, days of ventilated treatment, the volume of daily aspirated aubglottic secretions, the morbidity and timing of VAP, days of stay in ICU and mortality within 28 days of hospitalization were recorded. Results One hundred and one patients were included in the study. There were 48 patients in the CASS group who were treated with mechanical ventilation more than 48 hours,and 43 patients in the NASS group. There was no significant difference in the general status of the patients and days of ventilation between 2 groups with the averaged score of APACHE Ⅱ being 20.8± 6.1. The average of CPIS was of 5.6±1.0 when VAP was diagnosed. The mean volume of aspirated subglottic secretions within the first 24 hours in the CASS group (n=48) was (27.2±21.2)ml. The morbidity of VAP in the CASS and the NASS groups was 25.0% and 46. 5% respectively (P=0.032), and the length of time before the onset of VAP in these 2 groups was (7.3±4.2) days and (5.1±3.0) days respectively (P=0.100). There was a significant increase in the percentage of gram-positive cocci from the lower respiratory tracts in the NASS group compared with that in the CASS group (P=0.004). In the CASS group, the volume of the first daily aspirated subglottic secretions in patients with VAP was significantly less than that in patients without VAP(P =0.006). The morbidity of VAP in patients with failed early aspiration (the volume of first daily aspirated secretions≤20 ml) was significantly higher than that in patients in whom the aspiration was effective (P<0.01). The length of mechanical ventilation in patients with VAP was significantly longer than that in patients without VAP(P=0.000). The in-hospital mortality in patients with VAP was significantly higher than that in patients without VAP(P=0.009), and the mortality in 28 days after admission in patients with VAP was significantly higher than that in patients without VAP(P=0.035).Conclusion Effective continuous aspiration of subglottic secretions could significantly reduce the morbidity of early-onset VAP.