We studied 21 intubated premature infants (wts 800-2800 g) with respiratory distress syndrome between day 2 and 10 to evaluate the effect of body position on lung mechanics and gas exchange. The dynamic compliance of the total respiratory system was similar in the prone and supine position. When the infant was turned from the supine or the prone position to the other one, a significant improvement of oxygenation was seen temporarily. Positioning did not significantly affect the dynamic compliance, the minute volume or pCO2.. In circulatory stable premature infants a change of the body position probably alters the regional ventilation to perfusion ratio and leads to a reduction of intrapulmonary venous admixture.