['In the semirecumbent position, gravity-dependent dissemination of pathogens has been implicated in the pathogenesis of ventilator-associated pneumonia. We compared the preventive effects of a ventilatory strategy, aimed at decreasing pulmonary aspiration and enhancing mucus clearance versus the Trendelenburg position.', 'Prospective randomized animal study.', 'Animal research facility, University of Barcelona, Spain.', 'Twenty-four Large White-Landrace pigs.', 'Pigs were intubated and on mechanical ventilation for 72 hours. Following surgical preparation, pigs were randomized to be positioned: 1) in semirecumbent/prone position, ventilated with a duty cycle (TITTOT) of 0.33 and without positive end-expiratory pressure (control); 2) as in the control group, positive end-expiratory pressure of 5\u2009cm H2O and TITTOT to achieve a mean expiratory-inspiratory flow bias of 10\u2009L/min (treatment); 3) in Trendelenburg/prone position and ventilated as in the control group (Trendelenburg). Following randomization, Pseudomonas aeruginosa was instilled into the oropharynx.', 'Mucus clearance rate was measured through fluoroscopic tracking of tracheal markers. Microspheres were instilled into the subglottic trachea to assess pulmonary aspiration. Ventilator-associated pneumonia was confirmed by histological/microbiological studies. The mean expiratory-inspiratory flow in the treatment, control, and Trendelenburg groups were 10.7\u2009±\u20091.7, 1.8\u2009±\u20093.7 and 4.3\u2009±\u20092.8\u2009L/min, respectively (p < 0.001). Mucus clearance rate was 11.3\u2009±\u20099.9\u2009mm/min in the Trendelenburg group versus 0.1\u2009±\u20091.0 in the control and 0.2\u2009±\u20091.0 in the treatment groups (p = 0.002). In the control group, we recovered 1.35% ± 1.24% of the instilled microspheres per gram of tracheal secretions, whereas 0.22% ± 0.25% and 0.97% ± 1.44% were recovered in the treatment and Trendelenburg groups, respectively (p = 0.031). Ventilator-associated pneumonia developed in 66.67%, 85.71%, and 0% of the animals in the control, treatment, and Trendelenburg groups (p < 0.001).'] Does gravity predominate over ventilatory pattern in the prevention of ventilator-associated pneumonia?