Acute respiratory failure is still one the main problems in surgical intensive care. Unknown pathophysiological mechanisms permit only symptomatic therapy. Today ventilatory strategies by using PEEP und IRV are established to improve gas exchange and FRC by recruiting collapsed alveoli, decreasing intrapulmonary shunting and returning V/Q matching to normal. Furthermore different studies have shown the effects of supine and lateral decubitus posture in patients with acute respiratory failure. There are only rare reports on using the prone position, which doesn't require two-lung ventilation in difference to lateral position. We have studied 16 patients with acute respiratory failure by using continuous changing between prone and supine position under mechanical ventilation. All were male, aged 41.3 years in the middle and showed an average "Injury Severity Score" of 30 (13-50). 15 were trauma patients with blunt chest trauma in 11 cases. We have used prone position on threatening or manifest ARDS. In all patients we observed an increment of PaO2 during prone position on to 48 mmHg so that FiO2 could be reduced on an average of 0.2 within the first 48 h since changing patient's position. Posture changing depends on blood gas analysis, specifically on decreasing PaO2 after previous increment. Patients remained in prone and supine position at a mean of 6.3 (4.5-20) h and posture changing was proceeded over a period of 15.4 (7-32) days. No problems recording to blood pressure or mechanical ventilation appeared during prone position. 11 of 16 patients survived (68.8%), 5 died of cardiac (2) and multi organic failure (3) in connection with sepsis.(ABSTRACT TRUNCATED AT 250 WORDS)