BACKGROUND: Prone positioning has been adopted as a strategy to improve oxygenation in patients with refractory acute respiratory distress syndrome (ARDS). After returning to supine position, most of patients show arterial blood gas changes. However, the clinical implications have not been elucidated. This study was aimed to observe the relationship between the arterial blood gas changes followed by changing position from prone to supine and survival of ARDS. METHODS: We analyzed medical data of 53 ARDS patients, who showed improved arterial oxygenation (defined as the increase in PaO2/FiO2 by > or =20 mmHg within 8~12 hour after prone positioning) in a medical intensive care unit from January, 2000 to July, 2006. The patients were returned to supine position when they showed their PaO2/FiO2 > or =150 mmHg. We compared the arterial blood gas changes between the survivor and the nonsurvivor. RESULTS: The survivor has significant pH improvement after position change (the survivor 0.01+/-0.06 vs. the nonsurvivor -0.03+/-0.08; p=.03). The PaO2/FiO2 and FiO2 changes were not different between the survivor (14.44 +/-69.68 and -2.2+/-4.3, respectively) and the nonsurvivor (-7.17+/-83.94 and 1.8+/-6.0, respectively; p=.314 and .843). The patients whose PaO2/FiO2 were deteriorated had higher mortality without statistical significance (p=.305). The PaCO2 changes were not different between two groups (-0.05+/-11.46 vs. 3.47+/-17.62, p=.390). CONCLUSIONS: The early changes in pH differed significantly between the survivor and the nonsurvivor after returning patients to supine position from prone. Whether this marker can be a predictor of survival should be studied further.